Random access channel signal transmission method and user equipment, and random access channel signal reception method and base station

ABSTRACT

In a wireless communication system according to the present invention, a plurality of synchronization signals may be transmitted on a cell. The plurality of synchronization signals may be respectively associated with a plurality of random access channel establishments. A user equipment may receive at least one of the plurality of synchronization signals. The user equipment may transmit a random access channel using a random access channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/084,091, filed on Sep. 11, 2018, which is a National Stageapplication under 35 U.S.C. § 371 of International Application No.PCT/KR2017/002250, filed on Mar. 2, 2017, which claims the benefit ofU.S. Provisional Application No. 62/441,573, filed on Jan. 3, 2017, U.S.Provisional Application No. 62/349,078, filed on Jun. 12, 2016, U.S.Provisional Application No. 62/333,290, filed on May 9, 2016, U.S.Provisional Application No. 62/310,795, filed on Mar. 20, 2016, and U.S.Provisional Application No. 62/307,319, filed on Mar. 11, 2016. Thedisclosures of the prior applications are incorporated by reference intheir entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to methods and devices for transmitting/receivingrandom access channel signal.

BACKGROUND

With appearance and spread of machine-to-machine (M2M) communication anda variety of devices such as smartphones and tablet PCs and technologydemanding a large amount of data transmission, data throughput needed ina cellular network has rapidly increased. To satisfy such rapidlyincreasing data throughput, carrier aggregation technology, cognitiveradio technology, etc. for efficiently employing more frequency bandsand multiple input multiple output (MIMO) technology, multi-base station(BS) cooperation technology, etc. for raising data capacity transmittedon limited frequency resources have been developed.

A general wireless communication system performs datatransmission/reception through one downlink (DL) band and through oneuplink (UL) band corresponding to the DL band (in case of a frequencydivision duplex (FDD) mode), or divides a prescribed radio frame into aUL time unit and a DL time unit in the time domain and then performsdata transmission/reception through the UL/DL time unit (in case of atime division duplex (TDD) mode). A base station (BS) and a userequipment (UE) transmit and receive data and/or control informationscheduled on a prescribed time unit basis, e.g. on a subframe basis. Thedata is transmitted and received through a data region configured in aUL/DL subframe and the control information is transmitted and receivedthrough a control region configured in the UL/DL subframe. To this end,various physical channels carrying radio signals are formed in the UL/DLsubframe. In contrast, carrier aggregation technology serves to use awider UL/DL bandwidth by aggregating a plurality of UL/DL frequencyblocks in order to use a broader frequency band so that more signalsrelative to signals when a single carrier is used can be simultaneouslyprocessed.

In addition, a communication environment has evolved into increasingdensity of nodes accessible by a user at the periphery of the nodes. Anode refers to a fixed point capable of transmitting/receiving a radiosignal to/from the UE through one or more antennas. A communicationsystem including high-density nodes may provide a better communicationservice to the UE through cooperation between the nodes.

As more communication devices have demanded higher communicationcapacity, there has been necessity of enhanced mobile broadband (eMBB)relative to legacy radio access technology (RAT). In addition, massivemachine type communication (mMTC) for providing various services atanytime and anywhere by connecting a plurality of devices and objects toeach other is one main issue to be considered in next generationcommunication.

Further, a communication system to be designed in consideration of aservice/UE sensitive to reliability and standby time is underdiscussion. Introduction of next generation radio access technology hasbeen discussed by taking into consideration eMBB communication, mMTC,ultra-reliable and low-latency communication (URLLC), and the like.

SUMMARY

Due to introduction of new radio communication technology, the number ofuser equipments (UEs) to which a BS should provide a service in aprescribed resource region increases and the amount of data and controlinformation that the BS should transmit to the UEs increases. Since theamount of resources available to the BS for communication with the UE(s)is limited, a new method in which the BS efficiently receives/transmitsuplink/downlink data and/or uplink/downlink control information usingthe limited radio resources is needed.

With development of technologies, overcoming delay or latency has becomean important challenge. Applications whose performance criticallydepends on delay/latency are increasing. Accordingly, a method to reducedelay/latency compared to the legacy system is demanded.

Also, with development of smart devices, a new scheme for efficientlytransmitting/receiving a small amount of data or efficientlytransmitting/receiving data occurring at a low frequency is required.

In addition, a signal transmission/reception method is required in thesystem supporting new radio access technologies.

The technical objects that can be achieved through the present inventionare not limited to what has been particularly described hereinabove andother technical objects not described herein will be more clearlyunderstood by persons skilled in the art from the following detaileddescription.

In a wireless communication system according to the present invention, aplurality of synchronization signals may be transmitted on a cell. Theplurality of synchronization signals may be respectively associated witha plurality of random access channel configurations. A user equipmentmay receive at least one of the plurality of synchronization signals.The user equipment may transmit a random access channel using a randomaccess channel configuration associated with a synchronization signalthat the user equipment received.

In an aspect of the present invention, provided herein is a method fortransmitting a random access channel signal by a user equipment (UE).The method comprises: receiving a synchronization signal of a cell;synchronizing with the cell using the synchronization signal; andtransmitting a random access channel over the cell. The cell may includea plurality of synchronization signals. The plurality of synchronizationsignals may be respectively related to a plurality of random accesschannel configurations. The random access channel may be transmittedusing a random access channel configuration related to thesynchronization signal among the plurality of random access channelconfigurations.

In another aspect of the present invention, provided herein is a methodfor receiving a random access channel signal by a base station (BS). themethod comprises: transmitting a plurality of synchronization signalsover a cell; and receiving a random access channel of a user equipment(UE) on the cell. The plurality of synchronization signals may berespectively related to a plurality of random access channelconfigurations. The random access channel of the UE may be receivedusing one among the plurality of random access channel configurations.

In a further aspect of the present invention, provided herein is a userequipment (UE) for transmitting a random access channel signal. The UEmay include a radio frequency (RF) unit and a processor configured tocontrol the RF unit. The processor may be configured to: control the RFunit to receive a synchronization signal of a cell; synchronize with thecell using the synchronization signal; and control the RF unit totransmit a random access channel over the cell. The cell may include aplurality of synchronization signals. The plurality of synchronizationsignals may be respectively related to a plurality of random accesschannel configurations. the processor may be configured to control theRF unit to transmit the random access channel using a random accesschannel configuration related to the synchronization signal among theplurality of random access channel configurations.

In still another aspect of the present invention, provided herein is abase station (BS) for receiving a random access channel signal. The BSmay include a radio frequency (RF) unit and a processor configured tocontrol the RF unit. The processor may be configured to: control the RFunit to transmit a plurality of synchronization signals over a cell; andcontrol the RF unit to receive a random access channel of a userequipment (UE) on the cell. The plurality of synchronization signals maybe respectively related to a plurality of random access channelconfigurations. The random access channel of the UE may be receivedusing one among the plurality of random access channel configurations.

In each aspect of the present invention, configuration informationindicating the random access channel configuration related to thesynchronization signal may be transmitted by the BS or received by theUE.

In each aspect of the present invention, the configuration informationmay be included in in system information related to the synchronizationsignal to be transmitted by the BS or to be received by the UE.

In each aspect of the present invention, the plurality ofsynchronization signals may be distinguished by different indices otherthan a frame index, a subframe index and a symbol index.

In each aspect of the present invention, a random access response may bereceived by the UE or transmitted by the BS in response to the randomaccess channel. A downlink channel may be transmitted by the BS orreceived by the UE using an index indicated by the random accessresponse among the different indices. An uplink channel may be receivedby the BS or transmitted by the UE using an index indicated by therandom access response among the different indices.

The above technical solutions are merely some parts of the embodimentsof the present invention and various embodiments into which thetechnical features of the present invention are incorporated can bederived and understood by persons skilled in the art from the followingdetailed description of the present invention.

According to the present invention, uplink/downlink signals can beefficiently transmitted/received. Therefore, overall throughput of aradio communication system can be improved.

According to an embodiment of the present invention, delay/latencyoccurring during communication between a user equipment and a basestation may be reduced.

In addition, owing to development of smart devices, it is possible toefficiently transmit/receive not only a small amount of data but alsodata which occurs infrequently.

Moreover, signals can be transmitted/received in the system supportingnew radio access technologies.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIGS. 1A and 1B illustrate the structure of a radio frame used in theLTE/LTE-A based wireless communication system.

FIG. 2 illustrates the structure of a downlink (DL)/uplink (UL) slot inthe LTE/LTE-A based wireless communication system.

FIGS. 3A and 3B illustrate a radio frame structure for transmission of asynchronization signal (SS) slot in the LTE/LTE-A based wirelesscommunication system.

FIG. 4 illustrates the structure of a DL subframe used in the LTE/LTE-Abased wireless communication system.

FIG. 5 illustrates the structure of a UL subframe used in the LTE/LTE-Abased wireless communication system.

FIG. 6 illustrates an example of a short transmission time interval(TTI) and a transmission example of a control channel and a data channelin the short TTI.

FIG. 7 illustrates an application example of analog beamforming.

FIG. 8 illustrates a self-contained subframe structure.

FIGS. 9A and 9B illustrate examples of the time periods and resourceregions of the new system where primary synchronization signal(PSS)/secondary synchronization signal (SSS)/physical broadcast channel(PBCH) are transmitted.

FIGS. 10A and 10B illustrate examples of a method for transmittingsynchronization signals in the new system.

FIG. 11 illustrate an example where system information istransmitted/received according to the present invention.

FIG. 12 illustrates an example where an SS is transmitted per beamdirection on a cell or carrier.

FIGS. 13A and 13B illustrate random access response (RAR) messageformats according to the present invention.

FIG. 14 illustrates physical random access channel (PRACH) transmissionaccording to the present invention.

FIG. 15 is a block diagram illustrating elements of a transmittingdevice 10 and a receiving device 20 for implementing the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to the examples of the presentinvention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain examplesof the present invention, rather than to show the only examples that canbe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

In some instances, known structures and devices are omitted or are shownin block diagram form, focusing on important features of the structuresand devices, so as not to obscure the concept of the present invention.The same reference numbers will be used throughout this specification torefer to the same or like parts.

The following techniques, apparatuses, and systems may be applied to avariety of wireless multiple access systems. Examples of the multipleaccess systems include a code division multiple access (CDMA) system, afrequency division multiple access (FDMA) system, a time divisionmultiple access (TDMA) system, an orthogonal frequency division multipleaccess (OFDMA) system, a single carrier frequency division multipleaccess (SC-FDMA) system, and a multicarrier frequency division multipleaccess (MC-FDMA) system. CDMA may be embodied through radio technologysuch as universal terrestrial radio access (UTRA) or CDMA2000. TDMA maybe embodied through radio technology such as global system for mobilecommunications (GSM), general packet radio service (GPRS), or enhanceddata rates for GSM evolution (EDGE). OFDMA may be embodied through radiotechnology such as institute of electrical and electronics engineers(IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or evolved UTRA(E-UTRA). UTRA is a part of a universal mobile telecommunications system(UMTS). 3rd generation partnership project (3GPP) long term evolution(LTE) is a part of evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employsOFDMA in DL and SC-FDMA in UL. LTE-advanced (LTE-A) is an evolvedversion of 3GPP LTE. For convenience of description, it is assumed thatthe present invention is applied to 3GPP LTE/LTE-A. However, thetechnical features of the present invention are not limited thereto. Forexample, although the following detailed description is given based on amobile communication system corresponding to a 3GPP LTE/LTE-A system,aspects of the present invention that are not specific to 3GPP LTE/LTE-Aare applicable to other mobile communication systems.

For example, the present invention is applicable to contention basedcommunication such as Wi-Fi as well as non-contention basedcommunication as in the 3GPP LTE/LTE-A system in which an eNB allocatesa DL/UL time/frequency resource to a UE and the UE receives a DL signaland transmits a UL signal according to resource allocation of the eNB.In a non-contention based communication scheme, an access point (AP) ora control node for controlling the AP allocates a resource forcommunication between the UE and the AP, whereas, in a contention basedcommunication scheme, a communication resource is occupied throughcontention between UEs which desire to access the AP. The contentionbased communication scheme will now be described in brief. One type ofthe contention based communication scheme is carrier sense multipleaccess (CSMA). CSMA refers to a probabilistic media access control (MAC)protocol for confirming, before a node or a communication devicetransmits traffic on a shared transmission medium (also called a sharedchannel) such as a frequency band, that there is no other traffic on thesame shared transmission medium. In CSMA, a transmitting devicedetermines whether another transmission is being performed beforeattempting to transmit traffic to a receiving device. In other words,the transmitting device attempts to detect presence of a carrier fromanother transmitting device before attempting to perform transmission.Upon sensing the carrier, the transmitting device waits for anothertransmitting device which is performing transmission to finishtransmission, before performing transmission thereof. Consequently, CSMAcan be a communication scheme based on the principle of “sense beforetransmit” or “listen before talk”. A scheme for avoiding collisionbetween transmitting devices in the contention based communicationsystem using CSMA includes carrier sense multiple access with collisiondetection (CSMA/CD) and/or carrier sense multiple access with collisionavoidance (CSMA/CA). CSMA/CD is a collision detection scheme in a wiredlocal area network (LAN) environment. In CSMA/CD, a personal computer(PC) or a server which desires to perform communication in an Ethernetenvironment first confirms whether communication occurs on a networkand, if another device carries data on the network, the PC or the serverwaits and then transmits data. That is, when two or more users (e.g.PCs, UEs, etc.) simultaneously transmit data, collision occurs betweensimultaneous transmission and CSMA/CD is a scheme for flexiblytransmitting data by monitoring collision. A transmitting device usingCSMA/CD adjusts data transmission thereof by sensing data transmissionperformed by another device using a specific rule. CSMA/CA is a MACprotocol specified in IEEE 802.11 standards. A wireless LAN (WLAN)system conforming to IEEE 802.11 standards does not use CSMA/CD whichhas been used in IEEE 802.3 standards and uses CA, i.e. a collisionavoidance scheme. Transmitting devices always sense carrier of a networkand, if the network is empty, the transmitting devices wait fordetermined time according to locations thereof registered in a list andthen transmit data. Various methods are used to determine priority ofthe transmitting devices in the list and to reconfigure priority. In asystem according to some versions of IEEE 802.11 standards, collisionmay occur and, in this case, a collision sensing procedure is performed.A transmitting device using CSMA/CA avoids collision between datatransmission thereof and data transmission of another transmittingdevice using a specific rule.

In examples of the present invention described below, the term “assume”may mean that a subject to transmit a channel transmits the channel inaccordance with the corresponding “assumption”. This may also mean thata subject to receive the channel receives or decodes the channel in aform conforming to the “assumption”, on the assumption that the channelhas been transmitted according to the “assumption”.

In the present invention, puncturing a channel on a specific resourcemeans that the signal of the channel is mapped to the specific resourcein the procedure of resource mapping of the channel, but a portion ofthe signal mapped to the punctured resource is excluded in transmittingthe channel. In other words, the specific resource which is punctured iscounted as a resource for the channel in the procedure of resourcemapping of the channel, a signal mapped to the specific resource amongthe signals of the channel is not actually transmitted. The receiver ofthe channel receives, demodulates or decodes the channel, assuming thatthe signal mapped to the specific resource is not transmitted. On theother hand, rate-matching of a channel on a specific resource means thatthe channel is never mapped to the specific resource in the procedure ofresource mapping of the channel, and thus the specific resource is notused for transmission of the channel. In other words, the rate-matchedresource is not counted as a resource for the channel in the procedureof resource mapping of the channel. The receiver of the channelreceives, demodulates, or decodes the channel, assuming that thespecific rate-matched resource is not used for mapping and transmissionof the channel.

In the present invention, a user equipment (UE) may be a fixed or mobiledevice. Examples of the UE include various devices that transmit andreceive user data and/or various kinds of control information to andfrom a base station (BS). The UE may be referred to as a terminalequipment (TE), a mobile station (MS), a mobile terminal (MT), a userterminal (UT), a subscriber station (SS), a wireless device, a personaldigital assistant (PDA), a wireless modem, a handheld device, etc. Inaddition, in the present invention, a BS generally refers to a fixedstation that performs communication with a UE and/or another BS, andexchanges various kinds of data and control information with the UE andanother BS. The BS may be referred to as an advanced base station (ABS),a node-B (NB), an evolved node-B (eNB), a base transceiver system (BTS),an access point (AP), a processing server (PS), etc.

In the present invention, a node refers to a fixed point capable oftransmitting/receiving a radio signal through communication with a UE.Various types of eNBs may be used as nodes irrespective of the termsthereof. For example, a BS, a node B (NB), an e-node B (eNB), apico-cell eNB (PeNB), a home eNB (HeNB), a relay, a repeater, etc. maybe a node. In addition, the node may not be an eNB. For example, thenode may be a radio remote head (RRH) or a radio remote unit (RRU). TheRRH or RRU generally has a lower power level than a power level of aneNB. Since the RRH or RRU (hereinafter, RRH/RRU) is generally connectedto the eNB through a dedicated line such as an optical cable,cooperative communication between RRH/RRU and the eNB can be smoothlyperformed in comparison with cooperative communication between eNBsconnected by a radio line. At least one antenna is installed per node.The antenna may mean a physical antenna or mean an antenna port or avirtual antenna.

In the present invention, a cell refers to a prescribed geographicalarea to which one or more nodes provide a communication service.Accordingly, in the present invention, communicating with a specificcell may mean communicating with an eNB or a node which provides acommunication service to the specific cell. In addition, a DL/UL signalof a specific cell refers to a DL/UL signal from/to an eNB or a nodewhich provides a communication service to the specific cell. A nodeproviding UL/DL communication services to a UE is called a serving nodeand a cell to which UL/DL communication services are provided by theserving node is especially called a serving cell. Furthermore, channelstatus/quality of a specific cell refers to channel status/quality of achannel or communication link formed between an eNB or node whichprovides a communication service to the specific cell and a UE. The UEmay measure DL channel state received from a specific node usingcell-specific reference signal(s) (CRS(s)) transmitted on a CRS resourceand/or channel state information reference signal(s) (CSI-RS(s))transmitted on a CSI-RS resource, allocated by antenna port(s) of thespecific node to the specific node. Detailed CSI-RS configuration may beunderstood with reference to 3GPP TS 36.211 and 3GPP TS 36.331documents.

Meanwhile, a 3GPP LTE/LTE-A system uses the concept of a cell in orderto manage radio resources and a cell associated with the radio resourcesis distinguished from a cell of a geographic region.

A “cell” of a geographic region may be understood as coverage withinwhich a node can provide service using a carrier and a “cell” of a radioresource is associated with bandwidth (BW) which is a frequency rangeconfigured by the carrier. Since DL coverage, which is a range withinwhich the node is capable of transmitting a valid signal, and ULcoverage, which is a range within which the node is capable of receivingthe valid signal from the UE, depends upon a carrier carrying thesignal, the coverage of the node may be associated with coverage of the“cell” of a radio resource used by the node. Accordingly, the term“cell” may be used to indicate service coverage of the node sometimes, aradio resource at other times, or a range that a signal using a radioresource can reach with valid strength at other times.

Meanwhile, the 3GPP LTE-A standard uses the concept of a cell to manageradio resources. The “cell” associated with the radio resources isdefined by combination of downlink resources and uplink resources, thatis, combination of DL CC and UL CC. The cell may be configured bydownlink resources only, or may be configured by downlink resources anduplink resources. If carrier aggregation is supported, linkage between acarrier frequency of the downlink resources (or DL CC) and a carrierfrequency of the uplink resources (or UL CC) may be indicated by systeminformation. For example, combination of the DL resources and the ULresources may be indicated by linkage of system information block type 2(SIB2). The carrier frequency means a center frequency of each cell orCC. A cell operating on a primary frequency may be referred to as aprimary cell (Pcell) or PCC, and a cell operating on a secondaryfrequency may be referred to as a secondary cell (Scell) or SCC. Thecarrier corresponding to the Pcell on downlink will be referred to as adownlink primary CC (DL PCC), and the carrier corresponding to the Pcellon uplink will be referred to as an uplink primary CC (UL PCC). A Scellmeans a cell that may be configured after completion of radio resourcecontrol (RRC) connection establishment and used to provide additionalradio resources. The Scell may form a set of serving cells for the UEtogether with the Pcell in accordance with capabilities of the UE. Thecarrier corresponding to the Scell on the downlink will be referred toas downlink secondary CC (DL SCC), and the carrier corresponding to theScell on the uplink will be referred to as uplink secondary CC (UL SCC).Although the UE is in RRC-CONNECTED state, if it is not configured bycarrier aggregation or does not support carrier aggregation, a singleserving cell configured by the Pcell only exists.

3GPP LTE/LTE-A standards define DL physical channels corresponding toresource elements carrying information derived from a higher layer andDL physical signals corresponding to resource elements which are used bya physical layer but which do not carry information derived from ahigher layer. For example, a physical downlink shared channel (PDSCH), aphysical broadcast channel (PBCH), a physical multicast channel (PMCH),a physical control format indicator channel (PCFICH), a physicaldownlink control channel (PDCCH), and a physical hybrid ARQ indicatorchannel (PHICH) are defined as the DL physical channels, and a referencesignal and a synchronization signal are defined as the DL physicalsignals. A reference signal (RS), also called a pilot, refers to aspecial waveform of a predefined signal known to both a BS and a UE. Forexample, a cell-specific RS (CRS), a UE-specific RS (UE-RS), apositioning RS (PRS), and channel state information RS (CSI-RS) may bedefined as DL RSs. Meanwhile, the 3GPP LTE/LTE-A standards define ULphysical channels corresponding to resource elements carryinginformation derived from a higher layer and UL physical signalscorresponding to resource elements which are used by a physical layerbut which do not carry information derived from a higher layer. Forexample, a physical uplink shared channel (PUSCH), a physical uplinkcontrol channel (PUCCH), and a physical random access channel (PRACH)are defined as the UL physical channels, and a demodulation referencesignal (DM RS) for a UL control/data signal and a sounding referencesignal (SRS) used for UL channel measurement are defined as the ULphysical signals.

In the present invention, a physical downlink control channel (PDCCH), aphysical control format indicator channel (PCFICH), a physical hybridautomatic retransmit request indicator channel (PHICH), and a physicaldownlink shared channel (PDSCH) refer to a set of time-frequencyresources or resource elements (REs) carrying downlink controlinformation (DCI), a set of time-frequency resources or REs carrying acontrol format indicator (CFI), a set of time-frequency resources or REscarrying downlink acknowledgement (ACK)/negative ACK (NACK), and a setof time-frequency resources or REs carrying downlink data, respectively.In addition, a physical uplink control channel (PUCCH), a physicaluplink shared channel (PUSCH) and a physical random access channel(PRACH) refer to a set of time-frequency resources or REs carryinguplink control information (UCI), a set of time-frequency resources orREs carrying uplink data and a set of time-frequency resources or REscarrying random access signals, respectively. In the present invention,in particular, a time-frequency resource or RE that is assigned to orbelongs to PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH is referred to asPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH RE orPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH time-frequency resource,respectively. Therefore, in the present invention, PUCCH/PUSCH/PRACHtransmission of a UE is conceptually identical to UCI/uplink data/randomaccess signal transmission on PUSCH/PUCCH/PRACH, respectively. Inaddition, PDCCH/PCFICH/PHICH/PDSCH transmission of an eNB isconceptually identical to downlink data/DCI transmission onPDCCH/PCFICH/PHICH/PDSCH, respectively.

Hereinafter, OFDM symbol/subcarrier/RE to or for whichCRS/DMRS/CSI-RS/SRS/UE-RS/TRS is assigned or configured will be referredto as CRS/DMRS/CSI-RS/SRS/UE-RS/TRS symbol/carrier/subcarrier/RE. Forexample, an OFDM symbol to or for which a tracking RS (TRS) is assignedor configured is referred to as a TRS symbol, a subcarrier to or forwhich the TRS is assigned or configured is referred to as a TRSsubcarrier, and an RE to or for which the TRS is assigned or configuredis referred to as a TRS RE. In addition, a subframe configured fortransmission of the TRS is referred to as a TRS subframe. Moreover, asubframe in which a broadcast signal is transmitted is referred to as abroadcast subframe or a PBCH subframe and a subframe in which asynchronization signal (e.g. PSS and/or SSS) is transmitted is referredto a synchronization signal subframe or a PSS/SSS subframe. OFDMsymbol/subcarrier/RE to or for which PSS/SSS is assigned or configuredis referred to as PSS/SSS symbol/subcarrier/RE, respectively.

In the present invention, a CRS port, a UE-RS port, a CSI-RS port, and aTRS port refer to an antenna port configured to transmit a CRS, anantenna port configured to transmit a UE-RS, an antenna port configuredto transmit a CSI-RS, and an antenna port configured to transmit a TRS,respectively. Antenna ports configured to transmit CRSs may bedistinguished from each other by the locations of REs occupied by theCRSs according to CRS ports, antenna ports configured to transmit UE-RSsmay be distinguished from each other by the locations of REs occupied bythe UE-RSs according to UE-RS ports, and antenna ports configured totransmit CSI-RSs may be distinguished from each other by the locationsof REs occupied by the CSI-RSs according to CSI-RS ports. Therefore, theterm CRS/UE-RS/CSI-RS/TRS ports may also be used to indicate a patternof REs occupied by CRSs/UE-RSs/CSI-RSs/TRSs in a predetermined resourceregion. In the present invention, both a DMRS and a UE-RS refer to RSsfor demodulation and, therefore, the terms DMRS and UE-RS are used torefer to RSs for demodulation.

For terms and technologies which are not specifically described amongthe terms of and technologies employed in this specification, 3GPPLTE/LTE-A standard documents, for example, 3GPP TS 36.211, 3GPP TS36.212, 3GPP TS 36.213, 3GPP TS 36.321 and 3GPP TS 36.331 may bereferenced.

FIGS. 1A and 1B illustrate the structure of a radio frame used in awireless communication system.

Specifically, FIG. 1A illustrates an exemplary structure of a radioframe which can be used in frequency division multiplexing (FDD) in 3GPPLTE/LTE-A and FIG. 1B illustrates an exemplary structure of a radioframe which can be used in time division multiplexing (TDD) in 3GPPLTE/LTE-A.

Referring to FIGS. 1A and 1B, a 3GPP LTE/LTE-A radio frame is 10 ms(307,200 T_(s)) in duration. The radio frame is divided into 10subframes of equal size. Subframe numbers may be assigned to the 10subframes within one radio frame, respectively. Here, Ts denotessampling time where T_(s)=1/ (2048*15 kHz). Each subframe is 1 ms longand is further divided into two slots. 20 slots are sequentiallynumbered from 0 to 19 in one radio frame. Duration of each slot is 0.5ms. A time interval in which one subframe is transmitted is defined as atransmission time interval (TTI). Time resources may be distinguished bya radio frame number (or radio frame index), a subframe number (orsubframe index), a slot number (or slot index), and the like.

A TTI refers to an interval at which data may be scheduled. For example,the transmission opportunity of a UL grant or DL grant is given every 1ms in the current LTE/LTE-A system. The UL/DL grant opportunity is notgiven several times within a time shorter than 1 ms. Accordingly, theTTI is 1 ms in the current LTE-LTE-A system.

A radio frame may have different configurations according to duplexmodes. In FDD mode for example, since DL transmission and ULtransmission are discriminated according to frequency, a radio frame fora specific frequency band operating on a carrier frequency includeseither DL subframes or UL subframes. In TDD mode, since DL transmissionand UL transmission are discriminated according to time, a radio framefor a specific frequency band operating on a carrier frequency includesboth DL subframes and UL subframes.

FIG. 2 illustrates the structure of a DL/UL slot structure in theLTE/LTE-A based wireless communication system.

Referring to FIG. 2 , a slot includes a plurality of orthogonalfrequency division multiplexing (OFDM) symbols in the time domain andincludes a plurality of resource blocks (RBs) in the frequency domain.The OFDM symbol may refer to one symbol duration. Referring to FIG. 2 ,a signal transmitted in each slot may be expressed by a resource gridincluding N^(DL/UL) _(RB)*N^(RB) _(sc) subcarriers and N^(DL/UL) _(symb)OFDM symbols. N^(DL) _(RB) denotes the number of RBs in a DL slot andN^(UL) _(RB) denotes the number of RBs in a UL slot. N^(DL) _(RB) andN^(UL) _(RB) depend on a DL transmission bandwidth and a UL transmissionbandwidth, respectively. N^(DL) _(symb) denotes the number of OFDMsymbols in a DL slot, N^(UL) _(symb) denotes the number of OFDM symbolsin a UL slot, and N^(RB) _(sc) denotes the number of subcarriersconfiguring one RB.

An OFDM symbol may be referred to as an OFDM symbol, a single carrierfrequency division multiplexing (SC-FDM) symbol, etc. according tomultiple access schemes. The number of OFDM symbols included in one slotmay be varied according to channel bandwidths and CP lengths. Forexample, in a normal cyclic prefix (CP) case, one slot includes 7 OFDMsymbols. In an extended CP case, one slot includes 6 OFDM symbols.Although one slot of a subframe including 7 OFDM symbols is shown inFIG. 2 for convenience of description, examples of the present inventionare similarly applicable to subframes having a different number of OFDMsymbols. Referring to FIG. 2 , each OFDM symbol includes N^(DL/UL)_(RB)*N^(RB) _(sc) subcarriers in the frequency domain. The type of thesubcarrier may be divided into a data subcarrier for data transmission,a reference signal (RS) subcarrier for RS transmission, and a nullsubcarrier for a guard band and a DC component. The null subcarrier forthe DC component is unused and is mapped to a carrier frequency f₀ in aprocess of generating an OFDM signal or in a frequency up-conversionprocess. The carrier frequency is also called a center frequency f_(c).

FIGS. 3A and 3B illustrate a radio frame structure for transmission of asynchronization signal (SS) in the LTE/LTE-A based wirelesscommunication system. Specifically, FIGS. 3A and 3B illustrate a radioframe structure for transmission of an SS and a PBCH in frequencydivision duplex (FDD), wherein FIG. 3A illustrates transmissionlocations of an SS and a PBCH in a radio frame configured as a normalcyclic prefix (CP) and FIG. 3B illustrates transmission locations of anSS and a PBCH in a radio frame configured as an extended CP.

If a UE is powered on or newly enters a cell, the UE performs an initialcell search procedure of acquiring time and frequency synchronizationwith the cell and detecting a physical cell identity N^(cell) _(ID) ofthe cell. To this end, the UE may establish synchronization with the eNBby receiving synchronization signals, e.g. a primary synchronizationsignal (PSS) and a secondary synchronization signal (SSS), from the eNBand obtain information such as a cell identity (ID).

An SS will be described in more detail with reference to FIGS. 3A and3B. An SS is categorized into a PSS and an SSS. The PSS is used toacquire time-domain synchronization of OFDM symbol synchronization, slotsynchronization, etc. and/or frequency-domain synchronization and theSSS is used to acquire frame synchronization, a cell group ID, and/or CPconfiguration of a cell (i.e. information as to whether a normal CP isused or an extended CP is used). Referring to FIGS. 3A and 3B, each of aPSS and an SSS is transmitted on two OFDM symbols of every radio frame.More specifically, SSs are transmitted in the first slot of subframe 0and the first slot of subframe 5, in consideration of a global systemfor mobile communication (GSM) frame length of 4.6 ms for facilitationof inter-radio access technology (inter-RAT) measurement. Especially, aPSS is transmitted on the last OFDM symbol of the first slot of subframe0 and on the last OFDM symbol of the first slot of subframe 5 and an SSSis transmitted on the second to last OFDM symbol of the first slot ofsubframe 0 and on the second to last OFDM symbol of the first slot ofsubframe 5. A boundary of a corresponding radio frame may be detectedthrough the SSS. The PSS is transmitted on the last OFDM symbol of acorresponding slot and the SSS is transmitted on an OFDM symbolimmediately before an OFDM symbol on which the PSS is transmitted. Atransmit diversity scheme of an SS uses only a single antenna port andstandards therefor are not separately defined.

Referring to FIGS. 3A and 3B, upon detecting a PSS, a UE may discernthat a corresponding subframe is one of subframe 0 and subframe 5because the PSS is transmitted every 5 ms but the UE cannot discernwhether the subframe is subframe 0 or subframe 5. Accordingly, the UEcannot recognize the boundary of a radio frame only by the PSS. That is,frame synchronization cannot be acquired only by the PSS. The UE detectsthe boundary of a radio frame by detecting an SSS which is transmittedtwice in one radio frame with different sequences.

A UE, which has demodulated a DL signal by performing a cell searchprocedure using an SSS and determined time and frequency parametersnecessary for transmitting a UL signal at an accurate time, cancommunicate with an eNB only after acquiring system informationnecessary for system configuration of the UE from the eNB.

The system information is configured by a master information block (MIB)and system information blocks (SIBs). Each SIB includes a set offunctionally associated parameters and may be categorized into an MIB,SIB Type 1 (SIB1), SIB Type 2 (SIB2), and SIB3 to SIB17 according toincluded parameters.

The MIB includes most frequency transmitted parameters which areessential for initial access of the UE to a network of the eNB. The UEmay receive the MIB through a broadcast channel (e.g. a PBCH). The MIBincludes DL bandwidth (BW), PHICH configuration, and a system framenumber SFN. Accordingly, the UE can be explicitly aware of informationabout the DL BW, SFN, and PHICH configuration by receiving the PBCH.Meanwhile, information which can be implicitly recognized by the UEthrough reception of the PBCH is the number of transmit antenna ports ofthe eNB. Information about the number of transmit antennas of the eNB isimplicitly signaled by masking (e.g. XOR operation) a sequencecorresponding to the number of transmit antennas to a 16-bit cyclicredundancy check (CRC) used for error detection of the PBCH.

SIB1 includes not only information about time-domain scheduling of otherSIBs but also parameters needed to determine whether a specific cell issuitable for cell selection. SIB1 is received by the UE throughbroadcast signaling or dedicated signaling.

A DL carrier frequency and a system BW corresponding to the DL carrierfrequency may be acquired by the MIB that the PBCH carries. A UL carrierfrequency and a system BW corresponding to the UL carrier frequency maybe acquired through system information which is a DL signal. If nostored valid system information about a corresponding cell is present asa result of receiving the MIB, the UE applies a DL BW in the MIB to a ULBW until SIB2 is received. For example, the UE may recognize an entireUL system BW which is usable for UL transmission thereby throughUL-carrier frequency and UL-BW information in SIB2 by acquiring SIB2.

In the frequency domain, a PSS/SSS and a PBCH are transmitted only in atotal of 6 RBs, i.e. a total of 72 subcarriers, irrespective of actualsystem BW, wherein 3 RBs are in the left and the other 3 RBs are in theright centering on a DC subcarrier on corresponding OFDM symbols.Therefore, the UE is configured to detect or decode the SS and the PBCHirrespective of DL BW configured for the UE.

After initial cell search, the UE may perform a random access procedureto complete access to the eNB. To this end, the UE may transmit apreamble through a physical random access channel (PRACH) and receive aresponse message to the preamble through a PDCCH and a PDSCH. Incontention based random access, the UE may perform additional PRACHtransmission and a contention resolution procedure of a PDCCH and aPDSCH corresponding to the PDCCH.

After performing the aforementioned procedure, the UE may performPDCCH/PDSCH reception and PUSCH/PUCCH transmission as generaluplink/downlink transmission procedures.

The random access procedure is also referred to as a random accesschannel (RACH) procedure. The random access procedure is used forvarious purposes including initial access, adjustment of ULsynchronization, resource assignment, and handover. Random accessprocedures are categorized into a contention-based procedure and adedicated (i.e., non-contention-based) procedure. The contention-basedrandom access procedure is used for general operations including initialaccess, while the dedicated random access procedure is used for limitedoperations such as handover. In the contention-based random accessprocedure, the UE randomly selects a RACH preamble sequence.Accordingly, it is possible for multiple UEs to transmit the same RACHpreamble sequence at the same time. Thereby, a contention resolutionprocedure needs to be subsequently performed. On the other hand, in thededicated random access procedure, the UE uses an RACH preamble sequencethat the eNB uniquely allocates to the UE. Accordingly, the randomaccess procedure may be performed without collision with other UEs.

The contention-based random access procedure includes the following foursteps. Messages transmitted in Steps 1 to 4 given below may be referredto as Msg1 to Msg4.

Step 1: RACH preamble (via PRACH) (from UE to eNB)

Step 2: Random access response (RAR) (via PDCCH and PDSCH) (from eNB toUE)

Step 3: Layer 2/layer 3 message (via PUSCH) (from UE to eNB)

Step 4: Contention resolution message (from eNB to UE)

The dedicated random access procedure includes the following threesteps. Messages transmitted in Steps 0 to 2 may be referred to as Msg0to Msg2, respectively. Uplink transmission (i.e., Step 3) correspondingto the RAR may also be performed as a part of the random accessprocedure. The dedicated random access procedure may be triggered usinga PDCCH for ordering transmission of an RACH preamble (hereinafter, aPDCCH order).

Step 0: RACH preamble assignment (from eNB to UE) through dedicatedsignaling

Step 1: RACH preamble (via PRACH) (from UE to eNB)

Step 2: RAR (via PDCCH and PDSCH) (from eNB to UE)

After transmitting the RACH preamble, the UE attempts to receive an RARwithin a preset time window. Specifically, the UE attempts to detect aPDCCH with RA-RNTI (Random Access RNTI) (hereinafter, RA-RNTI PDCCH)(e.g., CRC is masked with RA-RNTI on the PDCCH) in the time window. Indetecting the RA-RNTI PDCCH, the UE checks the PDSCH for presence of anRAR directed thereto. The RAR includes timing advance (TA) informationindicating timing offset information for UL synchronization, UL resourceallocation information (UL grant information), and a temporary UEidentifier (e.g., temporary cell-RNTI (TC-RNTI)). The UE may perform ULtransmission (of, e.g., Msg3) according to the resource allocationinformation and the TA value in the RAR. HARQ is applied to ULtransmission corresponding to the RAR. Accordingly, after transmittingMsg3, the UE may receive acknowledgement information (e.g., PHICH)corresponding to Msg3.

A random access preamble, i.e., a RACH preamble consists of a cyclicprefix (CP) having a length of T_(CP) and a sequence part having alength of T_(SEQ). T_(CP) and T_(SEQ) depend on a frame structure and arandom access configuration, and preamble formats are controlled byhigher layers. The RACH preamble is transmitted in a UL subframe.Transmission of random access preambles is restricted to be performed oncertain time and frequency resources. Such a resource is referred to asa PRACH resource. PRACH resources are numbered as the subframe numberincreases in a radio frame and the RPB number increases in the frequencydomain so that index 0 may correspond to the lowest PRB and subframe inthe radio frame. In addition, random access resources are definedaccording to a PRACH configuration index (cf. 3GPP TS 36.211). The PRACHconfiguration index is provided through a higher layer signal(transmitted from an eNB).

FIG. 4 illustrates the structure of a DL subframe used in the LTE/LTE-Abased wireless communication system.

Referring to FIG. 4 , a DL subframe is divided into a control region anda data region in the time domain. Referring to FIG. 4 , a maximum of 3(or 4) OFDM symbols located in a front part of a first slot of asubframe corresponds to the control region. Hereinafter, a resourceregion for PDCCH transmission in a DL subframe is referred to as a PDCCHregion. OFDM symbols other than the OFDM symbol(s) used in the controlregion correspond to the data region to which a physical downlink sharedchannel (PDSCH) is allocated. Hereinafter, a resource region availablefor PDSCH transmission in the DL subframe is referred to as a PDSCHregion.

Examples of a DL control channel used in 3GPP LTE include a physicalcontrol format indicator channel (PCFICH), a physical downlink controlchannel (PDCCH), a physical hybrid ARQ indicator channel (PHICH), etc.

The control information transmitted through the PDCCH will be referredto as downlink control information (DCI). The DCI includes resourceallocation information for a UE or UE group and other controlinformation. Transmit format and resource allocation information of adownlink shared channel (DL-SCH) are referred to as DL schedulinginformation or DL grant. Transmit format and resource allocationinformation of an uplink shared channel (UL-SCH) are referred to as ULscheduling information or UL grant. The size and usage of the DCIcarried by one PDCCH are varied depending on DCI formats. The size ofthe DCI may be varied depending on a coding rate. In the current 3GPPLTE system, various formats are defined, wherein formats 0 and 4 aredefined for a UL, and formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3 and 3Aare defined for a DL. Combination selected from control information suchas a hopping flag, RB allocation, modulation coding scheme (MCS),redundancy version (RV), new data indicator (NDI), transmit powercontrol (TPC), cyclic shift, cyclic shift demodulation reference signal(DM RS), UL index, channel quality information (CQI) request, DLassignment index, HARQ process number, transmitted precoding matrixindicator (TPMI), precoding matrix indicator (PMI) information istransmitted to the UE as the DCI.

The PDCCH is transmitted on an aggregation of one or a plurality ofcontinuous control channel elements (CCEs). The CCE is a logicallocation unit used to provide a coding rate based on the status of aradio channel to the PDCCH. The CCE corresponds to a plurality ofresource element groups (REGs). For example, each CCE includes nineREGs, and the nine REGs are distributed over first one/two/three OFDMsymbols (or four OFDM symbols if necessary for 1.4 MHz) and over thesystem bandwidth in order to mitigate interference for the purpose ofdiversity. One REG corresponds to four REs. Four QPSK symbols are mappedto each REG. A resource element (RE) occupied by the reference signal(RS) is not included in the REG. Accordingly, the number of REGs withingiven OFDM symbols is varied depending on the presence of the RS. TheREGs are also used for other downlink control channels (that is, PDFICHand PHICH).

FIG. 5 illustrates the structure of a UL subframe used in the LTE/LTE-Abased wireless communication system.

Referring to FIG. 5 , a UL subframe may be divided into a data regionand a control region in the frequency domain. One or several PUCCHs maybe allocated to the control region to deliver UCI. One or several PUSCHsmay be allocated to the data region of the UE subframe to carry userdata.

In the UL subframe, subcarriers distant from a direct current (DC)subcarrier are used as the control region. In other words, subcarrierslocated at both ends of a UL transmission BW are allocated to transmitUCI. ADC subcarrier is a component unused for signal transmission and ismapped to a carrier frequency f₀ in a frequency up-conversion process. APUCCH for one UE is allocated to an RB pair belonging to resourcesoperating on one carrier frequency and RBs belonging to the RB pairoccupy different subcarriers in two slots. The PUCCH allocated in thisway is expressed by frequency hopping of the RB pair allocated to thePUCCH over a slot boundary. If frequency hopping is not applied, the RBpair occupies the same subcarriers.

Recently, machine type communication (MTC) has come to the fore as asignificant communication standard issue. MTC refers to exchange ofinformation between a machine and an eNB without involving persons orwith minimal human intervention. For example, MTC may be used for datacommunication for measurement/sensing/reporting such as meter reading,water level measurement, use of a surveillance camera, inventoryreporting of a vending machine, etc. and may also be used for automaticapplication or firmware update processes for a plurality of UEs. In MTC,the amount of transmission data is small and UL/DL data transmission orreception (hereinafter, transmission/reception) occurs occasionally. Inconsideration of such properties of MTC, it would be better in terms ofefficiency to reduce production cost and battery consumption of UEs forMTC (hereinafter, MTC UEs) according to data transmission rate. Sincethe MTC UE has low mobility, the channel environment thereof remainssubstantially the same. If an MTC UE is used for metering, reading of ameter, surveillance, and the like, the MTC UE is very likely to belocated in a place such as a basement, a warehouse, and mountain regionswhich the coverage of a typical eNB does not reach. In consideration ofthe purposes of the MTC UE, it is better for a signal for the MTC UE tohave wider coverage than the signal for the conventional UE(hereinafter, a legacy UE).

When considering the usage of the MTC UE, there is a high probabilitythat the MTC UE requires a signal of wide coverage compared with thelegacy UE. Therefore, if the eNB transmits a PDCCH, a PDSCH, etc. to theMTC UE using the same scheme as a scheme of transmitting the PDCCH, thePDSCH, etc. to the legacy UE, the MTC UE has difficulty in receiving thePDCCH, the PDSCH, etc. Therefore, the present invention proposes thatthe eNB apply a coverage enhancement scheme such as subframe repetition(repetition of a subframe with a signal) or subframe bundling upontransmission of a signal to the MTC UE having a coverage issue so thatthe MTC UE can effectively receive a signal transmitted by the eNB. Forexample, the PDCCH and/or the PDSCH may be transmitted to the MTC UEhaving the coverage issue through multiple (e.g., about 100) subframes.

The examples of the present invention can be applied to not only the3GPP LTE/LTE-A system but also a new radio access technology (RAT)system. As a number of communication devices have required much highercommunication capacity, the necessity of mobile broadband communication,which is much enhanced compared to the conventional RAT, has increased.In addition, massive MTC capable of providing various services anytimeand anywhere by connecting a number of devices or things to each otherhas been considered as a main issue in the next generation communicationsystem. Moreover, the design of a communication system capable ofsupporting services/UEs sensitive to reliability and latency has alsobeen discussed. That is, the introduction of the next generation RATconsidering the enhanced mobile broadband communication, massive MTC,Ultra-reliable and low latency communication (URLLC), etc. has beendiscussed. For convenience of description, the corresponding technologyis simply referred to as a new RAT in this specification.

In the next system of LTE-A, a method to reduce latency of datatransmission is considered. Packet data latency is one of theperformance metrics that vendors, operators and also end-users (viaspeed test applications) regularly measure. Latency measurements aredone in all phases of a radio access network system lifetime, whenverifying a new software release or system component, when deploying asystem and when the system is in commercial operation.

Better latency than previous generations of 3GPP RATs was oneperformance metric that guided the design of LTE. LTE is also nowrecognized by the end-users to be a system that provides faster accessto internet and lower data latencies than previous generations of mobileradio technologies.

However, with respect to further improvements specifically targeting thedelays in the system little has been done. Packet data latency isimportant not only for the perceived responsiveness of the system; it isalso a parameter that indirectly influences the throughput. HTTP/TCP isthe dominating application and transport layer protocol suite used onthe internet today. According to HTTP Archive(http://httparchive.org/trends.php) the typical size of HTTP-basedtransactions over the internet are in the range from a few 10's ofKbytes up to 1 Mbyte. In this size range, the TCP slow start period is asignificant part of the total transport period of the packet stream.During TCP slow start the performance is latency limited. Hence,improved latency can rather easily be shown to improve the averagethroughput, for this type of TCP-based data transactions. In addition,to achieve really high bit rates (in the range of Gbps), UE L2 buffersneed to be dimensioned correspondingly. The longer the round trip time(RTT) is, the bigger the buffers need to be. The only way to reducebuffering requirements in the UE and eNB side is to reduce latency.

Radio resource efficiency could also be positively impacted by latencyreductions. Lower packet data latency could increase the number oftransmission attempts possible within a certain delay bound; hencehigher block error ration (BLER) targets could be used for the datatransmissions, freeing up radio resources but still keeping the samelevel of robustness for users in poor radio conditions. The increasednumber of possible transmissions within a certain delay bound, couldalso translate into more robust transmissions of real-time data streams(e.g. VoLTE), if keeping the same BLER target. This would improve theVoLTE voice system capacity.

There are more over a number of existing applications that would bepositively impacted by reduced latency in terms of increased perceivedquality of experience: examples are gaming, real-time applications likeVoLTE/OTT VoIP and video telephony/conferencing.

Going into the future, there will be a number of new applications thatwill be more and more delay critical. Examples include remotecontrol/driving of vehicles, augmented reality applications in e.g.smart glasses, or specific machine communications requiring low latencyas well as critical communications.

FIG. 6 illustrates an example of a short TTI and a transmission exampleof a control channel and a data channel in the short TTI.

To reduce a user plane (U-plane) latency to 1 ms, a shortened TTI (sTTI)shorter than 1 ms may be configured. For example, for the normal CP, ansTTI consisting of 2 OFDM symbols, an sTTI consisting of 4 OFDM symbolsand/or an sTTI consisting of 7 OFDM symbols may be configured.

In the time domain, all OFDM symbols constituting a default TTI or theOFDM symbols except the OFDM symbols occupying the PDCCH region of theTTI may be divided into two or more sTTIs on some or all frequencyresources in the frequency band of the default TTI.

In the following description, a default TTI or main TTI used in thesystem is referred to as a TTI or subframe, and the TTI having a shorterlength than the default/main TTI of the system is referred to as ansTTI. For example, in a system in which a TTI of 1 ms is used as thedefault TTI as in the current LTE/LTE-A system, a TTI shorter than 1 msmay be referred to as the sTTI. The method of transmitting/receiving asignal in a TTI and an sTTI according to embodiments described below isapplicable not only to the system according to the current LTE/LTE-Anumerology but also to the default/main TTI and sTTI of the systemaccording to the numerology for the new RAT environment.

In the downlink environment, a PDCCH for transmission/scheduling of datawithin an sTTI (i.e., sPDCCH) and a PDSCH transmitted within an sTTI(i.e., sPDSCH) may be transmitted. For example, referring to FIG. 6 , aplurality of the sTTIs may be configured within one subframe, usingdifferent OFDM symbols. For example, the OFDM symbols in the subframemay be divided into one or more sTTIs in the time domain. OFDM symbolsconstituting an sTTI may be configured, excluding the leading OFDMsymbols on which the legacy control channel is transmitted. Transmissionof the sPDCCH and sPDSCH may be performed in a TDM manner within thesTTI, using different OFDM symbol regions. In an sTTI, the sPDCCH andsPDSCH may be transmitted in an FDM manner, using different regions ofPRB(s)/frequency resources.

<OFDM Numerology>

The new RAT system uses an OFDM transmission scheme or a similartransmission scheme. For example, the new RAT system may follow the OFDMparameters defined in the following table.

TABLE 1 Parameter Value Subcarrier-spacing (Δƒ) 75 kHz OFDM symbollength 13.33 us Cyclic Prefix(CP) length 1.04 us/0/94 us System BW 100MHz No. of available subcarriers 1200 Subframe length 0.2 ms Number ofOFDM symbol per 14 symbols Subframe

<Analog Beamforming>

In millimeter wave (mmW), the wavelength is shortened, and thus aplurality of antenna elements may be installed in the same area. Forexample, a total of 100 antenna elements may be installed in a 5-by-5 cmpanel in a 30 GHz band with a wavelength of about 1 cm in a2-dimensional array at intervals of 0.5λ (wavelength). Therefore, inmmW, increasing the coverage or the throughput by increasing thebeamforming (BF) gain using multiple antenna elements is taken intoconsideration.

If a transceiver unit (TXRU) is provided for each antenna element toenable adjustment of transmit power and phase, independent beamformingis possible for each frequency resource. However, installing TXRU in allof the about 100 antenna elements is less feasible in terms of cost.Therefore, a method of mapping a plurality of antenna elements to oneTXRU and adjusting the direction of a beam using an analog phase shifteris considered. This analog beamforming method may only make one beamdirection in the whole band, and thus may not perform frequencyselective beamforming (BF), which is disadvantageous.

Hybrid BF with B TXRUs that are fewer than Q antenna elements as anintermediate form of digital BF and analog BF may be considered. In thecase of hybrid BF, the number of directions in which beams may betransmitted at the same time is limited to B or less, which depends onthe method of collection of B TXRUs and Q antenna elements.

FIG. 7 shows an example where analog beamforming is applied.

Referring to FIG. 7 , it is possible to transmit/receive signals bychanging beam directions over time.

The present invention describes an initial access procedure in themmWave system, which is different from the conventional one due to theanalog beamforming features. In addition, the present invention proposesnot only how a UE and an eNB operate according to the changed initialaccess procedure but signaling information that should be exchangedbetween the UE and eNB and a method therefor.

<Self-Contained Subframe Structure>

FIG. 8 illustrates a self-contained subframe structure.

In order to minimize the latency of data transmission in the TDD system,a self-contained subframe structure is considered in the newfifth-generation RAT.

In FIG. 8 , the hatched area represents the transmission region of a DLcontrol channel (e.g., PDCCH) carrying the DCI, and the black arearepresents the transmission region of a UL control channel (e.g., PUCCH)carrying the UCI. Here, the DCI is control information that the eNBtransmits to the UE. The DCI may include information on cellconfiguration that the UE should know, DL specific information such asDL scheduling, and UL specific information such as UL grant. The UCI iscontrol information that the UE transmits to the eNB. The UCI mayinclude a HARQ ACK/NACK report on the DL data, a CSI report on the DLchannel status, and a scheduling request (SR).

In FIG. 8 , the region of symbols from symbol index 1 to symbol index 12may be used for transmission of a physical channel (e.g., a PDSCH)carrying downlink data, or may be used for transmission of a physicalchannel (e.g., PUSCH) carrying uplink data. According to theself-contained subframe structure, DL transmission and UL transmissionmay be sequentially performed in one subframe, and thustransmission/reception of DL data and reception/transmission of ULACK/NACK for the DL data may be performed in one subframe. As a result,the time taken to retransmit data when a data transmission error occursmay be reduced, thereby minimizing the latency of final datatransmission.

In such a self-contained subframe structure, a time gap is needed forthe process of switching from the transmission mode to the receptionmode or from the reception mode to the transmission mode of the eNB andUE. On behalf of the process of switching between the transmission modeand the reception mode, some OFDM symbols at the time of switching fromDL to UL in the self-contained subframe structure are set as a guardperiod (GP).

Referring to FIG. 8 , in a wideband system, a DL control channel can beTime Division Multiplexed (TDMed) with DL data or UL data and thentransmitted. In this case, although an eNB may transmit a DL controlchannel(s) over the entire band, but one UE may receive its DL controlchannel in specific partial band rather than the entire band. In thiscase, the DL control channel corresponding to information transmittedfrom the eNB to the UE may contain not only DL specific information suchas DL scheduling but also information on a cell configuration and ULspecific information such as UL grant.

For example, it is expected that the new RAT system, which is called themmWave system or 5G system, will use wide system bandwidth.Specifically, depending on the frequency band, the minimum systembandwidth of 5 MHz, 10 MHz, 40 MHz, 80 MHz, etc. should be able to besupported. The minimum system band may vary according to the basicsubcarrier spacing. For example, when the basic subcarrier spacing isrespectively set to 15 kHz, 30 kHz, 120 kHz, and 240 kHz, the minimumsystem band may be 5 MHz, 10 MHz, 40 MHz, and 80 MHz, respectively. Forexample, the new RAT system is designed such that it operates on notonly 6 GHz or less but 6 GHz or more and a plurality of subcarriers areused in one system to support various scenarios and use cases. When thelength of a subcarrier is changed, the length of a subframe canincrease/decrease according to the change in the subcarrier length. Forexample, one subframe may be defined to have a short time period, forexample, 0.5 ms, 0.25 ms, 0.125 ms, etc. It is expected that the new RATsystem will use high frequency band (e.g., 6 GHz or higher) and supporta subcarrier spacing greater than 15 kHz, i.e., the subcarrier spacingof the conventional LTE system. For example, assuming that thesubcarrier spacing is 60 kHz, one resource unit (RU) can be defined astwelve subcarriers in the frequency domain and one subframe in the timedomain.

To be associated with and served by a specific system, a UE should firstperform the following operations. The UE should obtain the time andfrequency synchronization of the corresponding system, receive basicSystem Information (SI), and adjust its uplink timing. In general, sucha procedure is referred to as an initial access procedure, and theinitial access procedure includes a synchronization procedure and anRACH procedure (i.e., random access procedure). Hereinafter, theabove-mentioned synchronization procedure of the LTE system is brieflysummarized for convenience of description.

>PSS: Symbol timing acquisition, frequency synchronization, and cell IDdetection within cell ID group (3 hypotheses).

>SSS: cell ID group detection (168 hypotheses), 10-ms frame boundarydetection, and cyclic prefix (CP) detection (2 hypotheses).

>PBCH decoding: antenna configuration, 40-ms timing detection, systeminformation, system bandwidth, etc.

That is, a UE obtains OFDM symbol timing and subframe timing as well asa cell ID based on a PSS and an SSS, performs descrambling and decodingof a PBCH using the cell ID, and then obtains important information ofthe corresponding system. The basic synchronization procedure of themmWave or new RAT system (hereinafter referred to as the mmWave/new RATsystem) is similar to the above-described procedure, but the PSS/SSStransmission/reception method of the mmWave/new RAT system issignificantly different from the conventional one.

FIGS. 9A and 9B illustrate examples of the time periods and resourceregions of the new system where PSS/SSS/PBCH are transmitted.Specifically, FIG. 9A shows an example of the PSS/SSS/PBCH transmissionperiod, and FIG. 9B shows an example of the PSS/SSS/ESS/PBCHtransmission period.

Referring to FIGS. 9A and 9B, when one subframe consists of 14 OFDMsymbols, PSS/SSS/PBCH can be transmitted in different directions perOFDM symbol. The number of beam directions can be selected within therange of 1 to N. In addition, the number of beams (or beam directions)can be dynamically determined according to frequency or by consideringcell interference. When detecting a PSS, a UE can obtain symbolsynchronization and a physical cell ID. Alternatively, the UE can obtaina cell ID by detecting a PSS and an SSS.

In the LTE/LTE-A system, PSS/SSS have been transmittedomni-directionally, whereas, in the mmWave system, a method by which aneNB performs beamforming by rotating beam directions omni-directionallyto transmit signals such as PSS/SSS/PBCH has been considered. That is,beam sweeping or beam scanning means transmitting and receiving signalsby rotating beam directions as described above. For example, assumingthat an eNB can support or have a maximum of N beam directions, the eNBcan transmit signals such as PSS/SSS/PBCH in each of the N beamdirections. In other words, the eNB transmits synchronization signalssuch as PSS/SSS/PBCH in each direction by sweeping the directions thatthe eNB can have or support. Alternatively, if the eNB can form N beams,one beam group may be composed of several beams. PSS/SSS/PBCH can betransmitted in each beam group. In this case, one beam group includesone or more beams.

<Synchronization Procedure in New System>

Hereinafter, the structures of synchronization and broadcasting signalsused in the new system will be described. When PSS/SSS/PBCH aretransmitted based on beam scanning, a UE can acquire system timing asfollows.

* Symbol/Subframe Timing and Cell ID Acquisition

To obtain information on symbol timing, a UE detects a PSS transmittedat a fixed location (for example, in 6 or x PRBs with respect to thecenter frequency) or a variable location. Similarly, the UE can obtainsubframe timing and/or frame timing by detecting an SSS transmitted at aknown location, that is, at a location relative to that of the PSStransmission resource. Then, by combining the PSS and the SSS, the UEcan obtain a cell ID. To prevent signals transmitted through the SSS andrelevant hypotheses from significantly increasing, the UE may obtain thesubframe timing by detecting an additional synchronization signal (e.g.,Extended Synchronization Signal (ESS)) transmitted from the eNB.

* The number of Beam RS Ports

When PSS/SSS/PBCH and ESS are transmitted per beam direction, a BeamReference Signal (BRS) can be transmitted for link quality measurementper beam direction. In other words, the BRS can be transmitted for thepurpose of RSRP/RRM/RLM measurement, and it can be used for neighborcell measurement. In addition, the BRS can be transmitted over theentire band to allow a UE to perform measurement over the entire band.For example, the BRS may be an RS transmitted over the entire band perantenna port for an analog beam direction in which the PSS/SSS aretransmitted. In this case, the UE should know information on the numberof ports used for BRS transmission and the location of resources perport on which the BRS is transmitted in advance. The number of portsused for the BRS transmission, that is, the number of BRS ports that theUE should measure at the corresponding time can be provided through theESS. For example, assuming that the number of maximum BRS ports is 8,the eNB can inform the UE of {1, 2, 4, 8}, {2, 4, 6, 8}, or othercombinations having different values through the ESS. Here, informingthe number of antenna ports through the ESS may mean that the UE shouldattempt to detect the number of antenna ports from the ESS based onmultiple hypotheses for the number of antenna ports. As another methodfor informing the number of BRS ports, the number of ports for a BRSthat is transmitted in the same beam direction as a PBCH can be signaledon the PBCH. When the information on the BRS ports is transmitted on thePBCH, the UE should decode a PBCH of a neighbor cell and measure a BRSof the corresponding neighbor cell for neighbor cell measurement.

* Extended Synchronization Signal/Sequence (ESS)

As described above, an ESS can be transmitted on frequency resourcesdifferent from those used for transmitting PSS/SSS within the samesymbol. When obtaining a cell ID and symbol timing, a UE may obtainsubframe timing and frame timing from an ESS. Here, the acquisition ofsubframe timing may mean obtaining the start location of a subframe,that is, information indicating how many symbols exist prior to thesymbol detected by the UE in the corresponding subframe. In addition,frame timing may be related to the transmission periodicity of asynchronization signal. After detecting a synchronization signal, the UEmay estimate the time required until the same synchronization signalarrives and be able to know how many subframes exist prior to a specificsubframe in the corresponding time interval. In addition, the number ofBRS ports can be indicated through the ESS. Moreover, information on thenumber of DM-RS ports in a PBCH can be also indicated by the ESS.Simply, assuming that the PBCH is transmitted in the same transmitdiversity scheme as that for SFBC, the number of PBCH DM-RS ports may belimited to 2. Alternatively, the number of PBCH DM-RS ports (forexample, 1, 2, 4, or 8) may be indicated by the ESS. Further, the ESSmay carry information on the system bandwidth or BRS transmissionbandwidth. In this case, for RSRP measurement, the UE may checkresources used for BRS transmission without decoding the PBCH.

* Sequence Generation

>ESS: Considering that a UE should be able to obtain subframe timingfrom ESS detection, an ESS sequence needs to be designed such that theUE can obtain information about how many symbols exist prior to thesymbol where an ESS is present in a corresponding subframe. Thus, an ESSsequence should be a function of the cell ID and the index of the OFDMsymbol in which a corresponding ESS is transmitted.

FIGS. 10A and 10B illustrate examples of a method for transmittingsynchronization signals in the new system.

When an eNB does not have time resources sufficient to transmitsynchronization signals for all beams in one subframe, the eNB maytransmit the synchronization signals in a plurality of subframes. Here,the synchronization signal means a signal transmitted from the eNB forsynchronization such as PSS/SSS/SSS. Assuming that the transmissionperiodicity of a Synchronization Signal (SS) is composed of P subframesand an eNB can generate B beams, the eNB can assume that B subframes arerequired to transmit all the SSs in respective directions of the Bbeams. FIGS. 10A and 10B show a case in which B is 2. For example, it isassumed two subframes are required to transmit all SSs in each beamdirection. In addition, in a period corresponding to P subframes, an SStransmitted in the first subframe of the two SS subframes is denoted bySS1, and an SS transmitted in the second subframe is denoted by SS2. SS1and SS2 may represent part of a set of SSs with multiple beam directionsor some SSs transmitted in different subframes. SSs may be divided intoSS1 in a specific subframe and SS2 in another subframe for transmissionthereof. SS1 and SS2 may be transmitted at a certain time interval asshown in FIG. 10A, or they may be transmitted in consecutive subframesas shown in FIG. 10B. The time required for the eNB to transmit next SS1and SS2 after transmitting current SS1 and SS2 can be defined as thesynchronization signal transmission periodicity. In other words, thetime required for the eNB to transmit synchronization signals in all ofits beam directions after transmitting synchronization signals in thesame beam directions can be defined as the synchronization signaltransmission periodicity. For example, referring to FIGS. 10A and 10B,the synchronization signal transmission periodicity may be 2T subframes.When synchronization signals are distributed over a plurality ofsubframes and then transmitted in the plurality of subframes,information indicating which synchronization signal is transmitted inwhich subframe should be included. That is, when synchronization signalsare distributedly transmitted, an ESS sequence may contain informationon how many subframes there are before the subframe in which thecorresponding ESS sequence is transmitted. To indicate how manysubframes there are before the subframe in which the ESS sequence istransmitted among the subframes where the synchronization signals aretransmitted or the subframes corresponding to the synchronization signalperiodicity by using the ESS sequence, the ESS sequence may be generatedas a function of the subframe index.

>BRS Sequence

Since a BRS is transmitted in a different beam direction according tosymbols, a BRS sequence is generated as a function of the cell ID andsymbol index and/or beam index. Alternatively, the BRS is generated as afunction of the BRS antenna port number and BRS transmission subframenumber.

>PBCH DM-RS Sequence

A PBCH DM-RS sequence is generated as a function of the cell ID, symbolindex, PBCH transmission subframe number, and DM-RS port number.

>PBCH Information Contents

A PBCH may contain information such as basic system information, systemframe number, the number of antenna ports, system bandwidth, etc.Additionally, the PBCH may contain information on PRACH configuration,time-frequency resources used for System Information Block (SIB) (i.e.,SI) transmission (or the periodicity of SIB transmission),time-frequency resources used for paging transmission, etc. The PRACHconfiguration information may be included in the SIB (i.e., SI). Theinformation on the time-frequency resources that can be used for the SIBand paging transmission may be independently signaled per beam direction(for example, per beam index). Upon obtaining information ontime-frequency resources that can be used for SIB and paging reception,a UE performs blind decoding for PDCCH detection in order to receive thecorresponding information in the subframes designated for the SIB andpaging reception. When a UE-specific Search Space (USS) and a CommonSearch Space (CSS) are configured for a UE, the UE expects that a commonchannel is transmitted in subframes for the SIB/paging and then performsblind decoding of both the USS and CSS because the information on theSIB/paging transmission corresponds to system information. The UE doesnot perform blind decoding (BD) of the CSS on other subframes except thesubframes for the SIB/paging. In this case, the CSS may mean a searchspace for all UEs in a cell. Alternatively, it may mean a Group-specificSearch Space (GSS) commonly allocated for a plurality of UEs rather thanall UEs in a cell. A group RNTI or group ID that a UE should read may bedetermined as a function of the beam ID having the same symbol or beamdirection as that of the PSS/SSS that the UE successfully receives andthe cell ID. Even if the configuration of the group ID is not separatelyprovided to the UE, the UE may determine the group ID after detectingthe beam ID and cell ID. Based on the group ID, the UE may receive anRNTI value used for receiving group common data and control information(for example, the corresponding RNTI value is signaled to the UE).Alternatively, the UE may use the group ID for scrambling or as a DM-RSscrambling ID.

* On-Demand SI

FIG. 11 illustrate an example where system information istransmitted/received according to the present invention.

Meanwhile, in the conventional LTE/LTE-A system, all applicable systeminformation (SI) is periodically broadcasted using physical resources ofa corresponding cell. To broadcast the SI, different mechanisms areused. For example, an MIB is transmitted on a BCH with a periodicity of40 ms, and SIB1 is transmitted on a DL-SCH with a periodicity of 80 ms,and other applicable SIBs are transmitted on a DL-SCH withtime-frequency domain scheduling by an SI-RNTI on a PDCCH. Each of theapplicable SIBs has a configurable periodicity and located within a timewindow. In some cases, SI corresponding to carrier aggregation and/orDual Connectivity (DC) is provided through dedicated signaling, which isa part of the Radio Resource Control (RRC) reconfiguration procedure fora (P)Scell for a configured UE. Such an SI broadcasting approach ismainly suitable for macro cell deployment. However, this approach maynot be optimal for other scenarios. That is, this broadcasting approachhas a disadvantage in that resources are wasted, for example, when thereare no or few UEs that camp on a cell, access to the system, and/or areinterested in specific types of SIBs. Another disadvantage of thebroadcasting approach is that when a UE obtains initial systeminformation, latency occurs due to periodic broadcasting of the SI. Forsome SIBs, the UE should wait for a period until a relevant SIB(s) istransmitted, and average delay corresponding to half of a configuredperiod is typically required before the UE determines whether to accessthe features of the corresponding system. A further disadvantage is thatas the system is developed and additional functions are added, theexpandability of the broadcasting approach is impacted. As the amount ofSI increases, the broadcasting approach requires more resources.Specifically, as new information messages are added, each of themessages may need to be broadcasted through a new time window. Sincethis means that the UE should wake up more frequently, it may affect thepower consumption of the UE. In particular, when the broadcastingapproach is applied to the new RAT system, there may be additionaldisadvantages. When the new RAT system is deployed in a high-frequencyband. The new RAT may require multiple beams to provide proper coveragewhen it is deployed in high frequency bands (e.g., more than 6 GHz). Inthis situation, if the broadcasting approach is used as in theconventional LTE/LTE-A system, that is, if SI is transmitted via each ofthe multiple beams, it may be inappropriate or inefficient. It isexpected that the new RAT deployment includes not only macro deploymentbut high-density cells with small coverage. In the case of cells withwide coverage, the broadcasting approach for all the applicable systeminformation is appropriate to provide functions of system access,camping, mobility, etc. On the other hand, in the case of cells withsmall coverage, it is desirable to provide the similar features andfunctions to those of a macro cell, but since a few UEs may exist incell coverage at a given time, these cells may be more suitable fordedicated transmission. If the new RAT system supports a short TTI, a UEcan obtain system information more rapidly using a dedicated signalcompared to waiting for system information broadcasting. This may berequired to support URLLC services properly. Further, in a high-densitydeployment, the broadcasting approach may increase interference levelsand affects the network power consumption.

Therefore, the present invention proposes on-demand SI to reduce theamount of broadcasted SI. The on-demand SI may be provided when it isrequested by a UE. In the new RAT system, some or all of the systeminformation may be on-demand SI. In other words, in the frequency bandto which the new RAT system is applied, all SI may be transmitted in anon-demand manner. Alternatively, some of the SI may be transmitted in analways-on manner, and the rest of the SI may be transmitted in anon-demand manner. The always-on SI may be common SI for all UEstransmitted in all beam directions of a corresponding cell. For example,SI essential for initial access may be periodically broadcasted as thealways-on SI, and except the broadcast minimum SI, the remaining SI maybe the on-demand SI. The on-demand SI may be UE-group specific SItransmitted in a specific beam direction.

An SI tag indicates whether there are changes in corresponding SImessages. To support on-demand SIBs or PBCH transmission, an SI tag maybe transmitted through a PBCH or an ESS. For example, referring to FIG.11 , when an SI tag is changed, a UE may request SIB transmissionthrough a RACH procedure (S1130). According to such an on-demandapproach, even when the network updates SIBs (i.e., SI), the UE may notread the SIBs in a predetermined cycle if the UE does not need theupdate. Meanwhile, when the UE requests the SI transmission through theRACH procedure, the UE should perform the RACH procedure based onout-of-date SI to obtain the updated SI because the UE does not have theupdated SI. When the UE performs the RACH procedure based on theout-of-date SIB, RACH resources available for the UE can be determinedin advance (S1110). For example, the system may designate specifictime/frequency or time/frequency/preambles for an SI request in advance.In particular, (in frequency band of 6 GHz or lower) RACH resources foran SI request may be shared by all UEs in a cell. When beamforming isapplied in frequency band of 6 GHz or higher, RACH resources for an SIrequest may be designated per beam direction, that is, per SS block.When the UE performs PRACH transmission using corresponding RACHresources (S1130), an eNB may perform the SIB transmission before orafter transmitting a Random Access Response (RAR) (S1150). In addition,when an SI tag is changed, the network can transmit only updated SIBsinstead of transmitting all SIBs. If the eNB detects an SI request fromSI-request RACH resources, the eNB may broadcast SI in a correspondingcell (in a corresponding beam direction) or transmit correspondingsystem information through the RAR for a corresponding preamble index.Alternatively, scheduling information on a time/frequency region inwhich corresponding system information is transmitted in response to anSI request may be included in the RAR for the SI request and thentransmitted. When a UE transmits an RACH for the purpose of requestingon-demand SIBs or PBCH transmission, the UE may inform the eNB that thecorresponding PRACH transmission is for requesting the SIBs through themessage carried by the PRACH. In other words, the UE may explicitlyinform, through the PRACH, that the purpose of the PRACH transmission isnot to transmit specific data in UL but to receive specific data in DL,and more particularly, to request system information. To transmit suchan explicit indication, the UE may set a field in the message carried bythe PRACH to a specific value or transmit a specific sequence on thePRACH. In other words, separate PRACH resources or sequences foron-demand SIBs or PBCH transmission can be reserved (by the eNB for aspecific UE or a specific beam direction) (S1110). Such an indicationmay be transmitted in Msg1 or Msg3. For example, when UL timing is notsynchronized, a UE may attempt PRACH transmission and transmit an SIrequest in Msg3.

* Beam Index Acquisition and RACH Resource Selection Criteria

FIG. 12 illustrates an example where an SS is transmitted per beamdirection on a cell or carrier. Although FIG. 12 shows that SS blocks,and more particular, a plurality of SS blocks for multiple beams areconsecutively transmitted, SS blocks for a set of beams may not beconsecutively transmitted on a corresponding cell/carrier as shown inFIG. 10A).

Assuming that an eNB can have N beam directions and transmit PSS/SSS ineach of the N beam directions, a UE may observe that the signal strengthvaries in each direction when detecting the PSS/SSS. The UE performsPSS/SSS detection with respect to PSS/SSS, which are transmitted persymbol in different directions, on a subframe in which the PSS/SSS aretransmitted (hereinafter referred to as a PSS/SSS subframe). In additionto the PSS/SSS, an ESS, a PBCH, and an RS for PBCH decoding (hereinafterreferred to as a PBCH DM-RS) are transmitted through beamforming.Therefore, from the above-described signals, the UE can obtain the beamdirection most suitable for the corresponding UE, that is, the beamdirection where signals are received with the best channel quality.Then, by reporting the optimal beam direction or the resources where thePSS/SSS/PBCH with the best channel quality are present to the eNB, theUE can receive the PDCCH/PDSCH transmitted via the optimal beam.Similarly, for the PUSCH/PUCCH transmitted from the UE, the eNB mayperform proper reception (RX) beamforming.

To identify a beam direction, an index or ID may be assigned to eachbeam (that is, each beam direction) or per beam group. A beam index maybe tied with a symbol index where PSS/SSS/PBCH and BRS are transmittedso that the beam index may be implicitly promised/defined between theeNB and UE. Alternatively, a beam index may be tied with a symbol indexwhere PSS/SSS/PBCH and BRS are transmitted and a BRS port number havingthe best reception quality in the corresponding symbol so that the beamindex may be promised/defined between the eNB and UE.

The signals transmitted in the same direction including PSS/SSS/PBCH canbe defined as one SS block. When there are multiple SS blocks, the SSblocks may be separately indexed to distinguish therebetween. A specificSS block may indicate the transmission direction of a DL signal/channelsuch as PSS/SSS/PBCH. For example, in a certain system, if PSS/SSS/PBCHare transmitted in 10 beam directions, the PSS/SSS/PBCH transmitted inthe same direction may be configured as one SS block. In addition, thecorresponding system could be interpreted to have 10 SS blocks. Since SSblocks are related to beam directions one-by-one, an SS block indexcould be interpreted to be a beam index.

After detection of a cell ID, subframe timing, and symbol timing, theindex of a specific beam may be implicitly identified by the symbolindex where the PSS/SSS corresponding to the cell ID are transmitted andthe BRS port index with the best reception quality in the correspondingsymbol. However, when PSS/SSS transmission periodicity is P (e.g., Psubframes) and an eNB needs B subframes to transmit all its beams, it isimpossible to identify a beam index by simply using a cell ID and asymbol index. Thus, in this case, to identify the beam index,information on a subframe where corresponding PSS/SSS are detected(e.g., information on how many subframes there are before the subframewhere the PSS/SSS are present) should be combined with the cell ID andsymbol index. On the contrary, the eNB and UE can explicitly exchange abeam index with each other via an SSS, ESS, BRS (beam RS), or PBCH.Here, the BRS means an RS transmitted over the entire band to estimate abeam direction transmitted per symbol.

According to the present invention, a beam index may mean an index withan explicit number or order for beam directions. Alternatively, in thepresent invention, a beam index may mean a specific beam direction of asymbol where PSS/SSS are transmitted rather than an index with anexplicit number or order for beam directions. In addition, in thepresent invention, a beam index may mean the direction of a beamtransmitted via a BRS port with the best reception quality among thedirections of specific beams transmitted from an eNB in a symbol wherePSS/SSS are transmitted. Moreover, in the present invention, a beamindex may mean the index of a beam group including multiple beamdirections and indicate a grouped beam direction having multiple beamdirections. Further, in the present invention, a beam index may mean anSS block index as described above. In determining the beam index withthe highest reception quality, a UE may inform the eNB that which beamdirection is most suitable for the corresponding UE without explicitlyreporting the beam index as described above. That is, if a RACH resourceis configured for and connected to each direction in which broadcastsignals such as PSS/SSS/PBCH are transmitted, the UE can inform the bestbeam direction without signaling the beam index separately. In otherwords, if a UE transmits a PRACH preamble using a specific RACHresource, the eNB may know that the beam direction optimized for the UEis the beam direction in which the PSS/SSS/PBCH connected to thespecific PACH resource are transmitted. The eNB can estimate the beamindex with the best reception quality from the perspective of thecorresponding UE, that is, the SS block index from the RACH resourceused by the UE to transmit the PRACH preamble.

In order for a UE to determine a beam index or SS block index suitablefor the UE or an SS block index, the following alternatives can beconsidered. The UE may select a set of preferred beams using one or anycombination of the following alternatives and list up a plurality ofRACH resource candidates associated with the corresponding beams.

>Alt1: After successfully detecting PSS/SSS or ESS, a UE selects a beamindex or SS block index where the received SINR of the PSS/SSS is equalto or more than a specific threshold.

>Alt2: As a method for using a PBCH DM-RS used for PBCH demodulation, aUE selects a beam direction (e.g., beam index) or SS block index wherethe Reference Signal Received Power (RSRP) of the PBCH DM-RS is equal toor more than a specific threshold before the PBCH demodulation.

>Alt3: A UE selects a beam index or SS block index where the RSRP of theBRS, which is transmitted in partial or the entire band, is equal to ormore than a specific threshold from among beams where the UEsuccessfully performs PSS/SSS detection and PBCH decoding.

>Alt4: In addition to Alt1, Alt2, or Alt3, if it is assumed that load orpriority information is transmitted via an ESS or PBCH, a UE may selecta beam or SS block index with low loads or high priority from amongbeams where received signals are greater than a threshold by using suchinformation. Alternatively, the UE may select the beam or SS block indexby combing the received signal quality and load/priority information.For example, Alt4 may be used to prevent downlink/uplink and RACHresources from being concentrated on a specific beam direction anddistribute downlink/uplink loads over multiple beam directions.

>Alt5: A UE transmits information on multiple selected or detected beams(e.g., SS blocks) (for example, beam indices or SS block indices)through first uplink transmission corresponding PRACH Msg3 so that thenetwork can select a beam for the UE.

In each of Alt1, Alt2, Alt3, Alt4 and Alt5, when a UE selects theoptimal beam index or SS block index and reports correspondinginformation to the eNB, the UE may also transmit the hypothesis/basisused for selecting the corresponding beam index or SS block index. Forexample, a UE may report the received signal quality together with thebeam index or SS block. In Alt1, information on the received SINR of thePSS/SSS may be transmitted together with the corresponding beam index.In Alt 2, information on the RSRP of the PBCH DM-RS may be transmittedtogether with the corresponding beam index. In Alt3 or Alt4, if the beamor SS block is selected on the basis of a BRS, information on the RSRPof the BRS may be transmitted together with the corresponding beamindex. When the UE transmits a RACH preamble by selecting RACH resourcesassociated with the optimal beam direction instead of reporting the beamindex, the UE may report the quality of a DL signal received in thecorresponding beam direction while performing UL transmission after RACHmessage 3 (Msg3).

In addition, the UE may select a plurality of optimal beam indices or SSblock indices where the received signal quality is higher than apredetermined threshold and report to the network the selected DL beamdirections and information on the DL received signal quality per beamdirection. Specifically, the UE may report the information whentransmitting RACH Msg3 or transmitting UL data thereafter.

The above description of the SS block transmission in multiple beamdirections and mapping between SS blocks and RACH resources are made onthe premise that reciprocity capable of determiningtransmission/reception beam directions between a UE and a Transmissionand Reception Point (TRP) is established. However, in a multi-beamenvironment, PRACH preamble repetition or beam sweeping may beconsidered according to TX/RX reciprocal capability of a TRP (e.g., eNB)or UE. The TX/RX reciprocal capability could be interpreted as TX/RXbeam correspondence at the TRP and UE. If the TRP and UE cannot maintainthe TX/RX reciprocal capability in the multi-beam environment, the UEmay be unable to transmit an uplink signal in the beam direction wherethe UE receives a downlink signal. This is because the UL optimal pathmay be different from the DL optimal path. If the TRP can determine aTRP RX beam for corresponding uplink reception based on UE's downlinkmeasurements for one or more TX beams of the TRP and/or if the TRP candetermine a TRP TX beam for corresponding downlink transmission based onTRP's uplink measurements for one or more RX beams of the TRP, the TX/RXbeam correspondence at the TRP can be hold. Meanwhile, if the UE candetermine a UE RX beam for corresponding uplink transmission based onUE's downlink measurements for one or more RX beams of the UE and/or ifthe UE can determine a UE TX beam for corresponding downlink receptionbased on an indication from the TRP, which is made based on uplinkmeasurements for one or more TX beams of the UE, the TX/RX beamcorrespondence at the UE can be hold.

* PRACH Resource Configuration

According to the present invention, since PSS/SSS are transmittedthrough beamforming, RX beamforming should be applied to PRACHresources, which are used by a UE to attempt uplink random access,according to the direction in which an eNB transmits PSS/SSS forsuccessful PRACH reception. To this end, the PRACH resources may beallocated per PSS/SSS direction, that is, per beam index (or SS blockindex). As described above, a RACH resource may be associated with eachSS block index where PSS/SSS and PBCH are transmitted. Basically, a RACHresource may mean a time-frequency resource where an RACH preamble maybe transmitted.

When a UE successfully detects PSS/SSS and selects best N beams, the UEneeds to obtain information on PRACH resources per beam index or SSblock index. Basically, PRACH resources means time-frequency resourcesused by a UE to transmit a PRACH, and information on PRACH sequences,root sequences, PRACH transmission power, maximum retransmission number,repetition number, etc. may be further included. For example, the UE mayinform the network of its preference for a specific SS block index usingthe time-frequency resources used by the UE for PRACH preambletransmission and a PRACH preamble index used by the UE. The PRACHtime-frequency resources may include information on a subframe numberthat can be used to transmit a PRACH in the direction corresponding to abeam index (or value for indicating a corresponding subframe), symbolnumbers in the corresponding subframe, the number of symbols in thecorresponding subframe, a PRB index in the frequency domain (a value forindicating the location of a corresponding PRB in the frequency domain),and/or frequency-domain bandwidth, etc. PRACH resources for differentbeam indices or SS block indices may be TDMed. Assuming that an eNBperforms RX beam scanning to receive an RACH for a single beam, acorresponding beam direction is reserved during a corresponding symbolor beam scanning duration regardless of whether there is RACHtransmission. In other words, since the UE can autonomously transmit anRACH preamble on RACH resources, it is assumed that the correspondingRACH resources are reserved at all times. To reduce this period, anumber of UEs need to be multiplexed during a certain period by usingthe system bandwidth as much as possible. Thus, multiple PRACH resourcescan be configured in the frequency domain. UEs can be distinguished byusing Code Division Multiplexing (CDM), by using Frequency DivisionMultiplexing (FDM) instead of the CDM or by using both of the CDM andFDM. That is, even when UEs use the same time-frequency RACH resources,information indicating that a UE prefers a specific SS block index orinformation indicating that the signal quality of the corresponding SSblock index is good can be signaled if the UEs use different codes(e.g., preamble sequences).

As another method, PRACH resources can be commonly configured for allbeam indices. That is, time-frequency resources on which a UE cantransmit a PRACH and an eNB can expect PRACH transmission from a UE maybe commonly configured for all beam directions, or one time-frequencyresource may be allocated per beam index group. When PRACH resources arecommonly configured for multiple beams, latency can be reduced. WhenPRACH resources are commonly configured for multiple or all beams,PRACHs corresponding to a plurality of beam indices can be transmittedon one time-frequency resource. In other words, PRACH transmissiontime-frequency resources for UEs having different optimal beamdirections may be shared between multiple UEs or between multiple beamindices. Alternatively, PRACH time-frequency resources may be configuredin a cell-common manner. In the case of common PRACH resources, it isassumed that a UE transmits a PRACH without using a specific beam, andthe performance degradation caused when the eNB does not use RXbeamforming may be overcome by PRACH repeated transmission and the like.Alternatively, whether common PRACH resources or PRACH resource where RXbeams are assumed are selected can be determined by latencyrequirements, power constraint, RSRP, etc. In addition, how a UEtransmits a PRACH may be slightly changed according to RACH resourceselection. For example, in the case of transmission where no RX beam isassumed, the network may not know the beam optimized for the UE. In thiscase, there is a disadvantage in that a subsequent transmission channelsuch as an RAR may be transmitted without use of any TX beam. When suchcommon RACH resources are used, RACH resources may be fixedly reservedbased on beam directions, and thus the disadvantage that thecorresponding radio resources cannot be used by a UE with a differentbeam direction can be overcome. The network can dynamically adjust theamount of common RACH resources and the amount of RACH resources thatdepend on beam directions. In general, although the amount oftransmission resources through TX/RX beams and coverage thereof may below, resources for wide beams or omni-direction transmission may bedivided. Depending on whether omni-direction resources or RX beamresources are used, the eNB may differently configure the power of theUE or dynamically inform the UE of the resource type (e.g., PUSCHresource) so that the UE can operate by changing the number ofrepetitions. In other words, the eNB informs information on receivergain semi-statically or dynamically so that the UE can determine itspower and repetition. Alternatively, for better matching of thetransmission/reception beam directions between the UE and eNB, mutuallypromised RACH transmission resources may be distinguished from RACHtransmission resources used by the eNB to receive a PRACH preamble byrotating the reception beam direction. As a further example, for bettermatching of the transmission/reception beam directions between the UEand eNB, the mutually promised RACH transmission resources may bedistinguished from RACH transmission resources on which the eNB fixesthe reception direction but the UE performs PRACH preamble transmissionby rotating the transmission direction.

To compensate for the disadvantages of RAR transmission when the abovePRACH resources are used, the eNB may configure the beam direction wherean RAR is transmitted together with RACH resources. When a UE does notsupport the configured RAR beam direction, the UE may not select thecorresponding RACH resources. That is, although PRACH resources may beshared by all UEs, the PRACH resource may be sub-divided according to TXbeam directions of the RAR transmission, and each UE may select itsPRACH resources from among the sub-divided PRACH resources according tothe RAR transmission beam directions. That is, the network instructs theeNB to use TX beams as described above but may also allow the eNB toperform the RACH reception without the RX beam scanning. Alternatively,RACH resources may be configured such that the eNB can perform receptionbeam scanning instead of being fixed in the reception beam direction.

The above-described method by which RACH resources are shared by aplurality of SS block indices can be applied more effectively when thebeam correspondence of an eNB or TRP is not matched. When the TRP's beamcorrespondence is not matched, a UE may repeatedly transmit a PRACHpreamble, and the TRP may perform the reception beam scanning operationto receive the PRACH preamble. In this case, the preamble indices usedby the corresponding UE may be part of an index set associated with aspecific SS block index. That is, although the UE transmits a PRACHpreamble on time-frequency RACH resources shared by a plurality of SSblock indices, the UE may inform the network of its preferred specificSS block index because the PRACH preamble is related to the specific SSblock index. PRACH resource configuration information includes atime-frequency resource region that can be used by a UE for PRACHtransmission, a PRACH transmission preamble index, preamble transmissionpower, RA-RNTI information used for PRACH transmission. In addition, thePRACH resources may be separately configured per beam index or SS blockindex, and the information included in the PRACH configuration may beindependently configured per beam index or SS block. In other words, thepreamble index, preamble transmission power, and RA-RNIT may vary perbeam index or SS block index. All or some of the information included inthe PRACH configuration may be beam-index-specific. For example, thetime-frequency resources available for PRACH transmission may be sharedby a plurality of beam indices or all beam indices, and in this case,the PRACH time-frequency resources in the PRACH configuration may becommon for beam indices included in a specific beam index group or allbeam indices. However, each piece of information should be transmittedper beam index. In addition to the PRACH configuration, an RAR windowsize and time-frequency resource for RAR transmission may beconfigured/signaled per beam index (e.g., SS block index).Alternatively, the RAR window and/or time-frequency resources for theRAR transmission may be common for multiple beam indices.

When PRACH transmission has succeeded, the eNB transmits an RAR for thesuccessful PRACH transmission. In this case, RACH configurationinformation may include configuration information on the RAR, and it canbe provided to a UE(s). The RAR configuration informationrepresentatively contains information on a time-frequency region wherethe RAR is transmitted. Details will be described later. Information onthe PRACH resource configuration corresponding to a beam index can betransmitted on a channel different from that transmitted in a symbolwhere the PSS/SSS with the same beam index as that of the correspondingPRACH resources are present. The following options can be applied tochannels capable of carrying the PRACH configuration.

>Option 1. A PBCH carries the PRACH configuration: The PRACHconfiguration may be transmitted on a PBCH that is transmitted in thesame symbol and direction as those of the PSS/SSS (that is, with thesame beam index as that of the PSS/SSS). That is, if the PBCH is used tocarry the PRACH configuration, it may increase the amount of informationtransmitted on the PBCH. Considering that the PBCH should carryessential information that even a cell-edge UE needs to successfullydecode, this may not be an appropriate option. However, the amount ofresources for the PBCH transmission is sufficient, the PRACH informationcan be transmitted on the PBCH.

>Option 2. An SIB carries the PRACH configuration: An SIB containing thePRACH configuration as the main information may be transmitted in thesame symbol and direction as those of the PSS/SSS (that is, with thesame beam index as that of the PSS/SSS). Alternatively, the location ofresources for SIB transmission may be indicated by a PBCH. By receivingthe SIB at the corresponding location, a UE may obtain the PRACHconfiguration information of a corresponding beam index.

* PRACH Transmission and Reporting of Best N Beam Indices

A UE may select a plurality of beam indices/directions for the beamdirection that is preferred by the UE or has the best reception qualityand obtain PRACH configuration information per beam index or beamdirection. That is, an eNB may transmit the PRACH configurationinformation per beam index. In a multi-beam environment, a plurality ofSS blocks may be defined. Each of the plurality of SS blocks may betransmitted in its unique DL transmission beam direction. In addition,RACH resources may be configured per SS direction. The UE may receivesignals/channels in SS blocks and select an SS block index with the bestreception quality. Moreover, the UE may transmit a PRACH preamble byselecting the RACH resources associated with the corresponding SS blockindex. In this case, the UE may select one or more SS block indices andattempt the PRACH preamble transmission on RACH resources associatedwith each of the SS blocks. Hereinafter, options by which a UE transmitsa PRACH preamble will be described.

>Option 1. Sequential PRACH transmission: A UE attempts PRACHtransmission preferentially for the most preferred beam index (e.g., SSblock index) among preferred best N beam indices (e.g., SS blockindices). That is, the UE transmits a PRACH on a PRACH resourcecorresponding to the best beam index. Upon receiving PRACH Msg1 on aspecific PRACH resource, an eNB can understand that the corresponding UEprefers the beam corresponding to the PRACH resource. In addition, bytransmitting an RAR for the corresponding PRACH, the eNB may confirm (orapprove) the UE to use the corresponding beam index (i.e., correspondingbeam direction). The RAR is transmitted on a predefined RAR resource.When the UE fails to receive the RAR for the PRACH transmitted on thePRACH resource corresponding to the beam index (e.g., SS block index),the UE transmits a PRACH on a PRACH resource corresponding to the beamindex (e.g., SS block index) with the next best quality and waits for aRAR. The UE can report its preferred beam index by simply transmitting aPRACH corresponding to a specific beam index (e.g., SS block index). Inaddition, the eNB can confirm (or approve) use of the corresponding beamindex by transmitting an RAR in response to the corresponding PRACH orreject use of the corresponding beam index by transmitting no RAR. Whenthe UE does not receive any RAR for the transmitted PRACH, the UEattempts the PRACH transmission using a PRACH resource corresponding toanother beam index (e.g., SS block index) and waits for an RAR.

>Option 2. Multiple PRACH transmission: If a UE obtains PRACHconfiguration information corresponding to all best N beam indices(e.g., SS block indices), the UE may transmit a PRACH for each of aplurality of preferred PRACH configurations. In addition, an eNB maytransmit a RAR for each beam index. In other words, the UE mayseparately perform a RACH procure per beam index (e.g., SS block index).However, according to option 2, since one UE occupies a plurality ofPRACH resources for PRACH transmission, it has a disadvantage in thatcollision probability between UEs on PRACH resources increases.Meanwhile, the UE may transmit a plurality of PRACHs for different beamindices without receiving an RAR for the previously transmitted PRACH.The eNB may respond to a plurality of PRACHs transmitted from a specificUE using one RAR in a given RAR window. In this case, the eNB mayconfirm use of a specific beam index (e.g., SS block index) bytransmitting an RAR for the corresponding beam index. Specifically, theeNB may confirm use of the specific beam index by transmitting the RARon an RAR resource corresponding to the specific beam index ortransmitting the RAR using an RA-RNTI or sequence corresponding to thespecific beam index.

>Option 3. Preferred preamble sequence transmission: A UE may selectpreferred best N beam indices (e.g., SS block indices). Thereafter, whentransmitting a PRACH for a specific beam, the UE may report the mostpreferred beam index (e.g., SS block index) to an eNB. The UE mayperforms PRACH transmission by selecting the earliest PRACH resource inthe time domain or the most preferred PRACH resource, and in this case,the UE may selects a preamble index from the PRACH configurationcorresponding to the most preferred beam index. That is, a PRACHpreamble transmitted by the UE may be unrelated to the beam index of thePRACH resource used for transmitting the corresponding PRACH. In otherwords, if the UE knows in which direction the eNB performs RXbeamforming for the purpose of PRACH reception, the UE may transmit aPRACH on a specific PRACH resource in the corresponding direction, butthe UE may use a preamble index that is not related to the beam index(e.g., SS block index) associated with the corresponding resource butrelated to another beam index (e.g., SS block index) for the PRACHtransmission. By doing so, the eNB may know that the UE prefers the beamdirection corresponding to the beam index (e.g., SS block index) relatedto the corresponding beam index. Moreover, the eNB may also know thatthe beam index (i.e., beam direction) corresponding to the resourcesused for PRACH preamble transmission can be used fortransmission/reception to/from the corresponding UE.

>Option 4. PRACH transmission with preferred RA-RNTI: Similar to option3, a UE may transmit a PRACH on a PRACH resource corresponding to arandom beam index (e.g., SS block index) among best N beam directionssuitable for the corresponding UE but use RA-RNTI corresponding toanother beam index. Similar to option 3, the UE may have a differentpreference for PRACH resources and signal to an eNB that the channelenvironment is good by selecting RA-RNTI of another beam index (e.g., SSblock index). The RA-RNTI is a function of the PRACH transmission timeresource index (e.g., subframe number or symbol index), the frequencyresource index (e.g., PRB index or absolute frequency), and the beamindex.

>Option 5. PRACH transmission on common PRACH resource: The aboveoptions are described on the assumption that a PRACH resource isallocated/signaled per beam index (e.g., SS block index). However, theoptions can also be applied when a PRACH transmission time-frequencyresource are commonly allocated to all beam indices (e.g., SS blockindex) or when the resource is shared by a plurality of beam indexgroups. For example, an eNB may transmit a PRACH configuration per beam(i.e., per SS block index) or per beam group. In this case, part ofinformation on the PRACH configuration per beam index (e.g., SS blockindex) or per beam group may be beam index (e.g., SS block index) commoninformation, and other information may be beam index (e.g., SS blockindex) specific information. In particular, PRACH time-frequencyresources may be the beam index (e.g., SS block index) commoninformation. The UE transmits a PRACH on the signaled PRACHtime-frequency resource but the UE may use a preamble sequence/indexcorresponding to a specific beam index (e.g., SS block index) totransmit information on its preferred beam index (e.g., SS block index)to the eNB. In other words, a plurality of UEs may transmit PRACHs onthe PRACH time-frequency resource, but in this case, each UE may have adifferent preferred reception beam direction (i.e., transmission beamdirection of the eNB). Each UE may signal its preferred beam directionwhen transmitting a PRACH. For example, each UE may informs the eNB ofits preferred beam direction using a preamble index or RA-RNTI in aPRACH configuration corresponding to a specific beam index (e.g., SSblock index).

>Option 6. Earliest timing first: A UE may use the earliest PRACHresource among PRACH resources for candidate beams or available PRACHresources. If timing such as an RAR, Msg3, Msg4, etc. is used togetherwith a PRACH configuration, the UE may select a resource capable ofreducing the total process time maximally.

In addition to options 1 to 6, a method by which a UE selects a PRACHresource corresponding to the beam or SS block with the largest amountof resources or a method by which a UE selects a PRACH resource byconsidering the amount of loads as described above can be considered aswell. In other words, load information can be signaled as RACHconfiguration information per SS block. This load information may beused to restrict RACH transmission for a specific SS block index, andmore specifically, restrict to make a random access attempt using aspecific RACH resource. As a similar method, a transmission power offsetmay be signaled as RACH configuration information per SS block index.For example, when a UE measures a received signal level per beam (i.e.,per SS block), the UE may calculate an actual received signal level byadding or subtracting the signaled power offset. The power offset mayenforce a UE to select a specific SS block index and attempt a RACHprocedure by using an RACH resource associated with the selected SSblock or prohibit the UE from selecting the specific SS block index.

* RAR Configuration and RAR Transmission

In the LTE system, an RAR message includes an RACH preamble sequenceindex detected by an eNB, a Timing Advance (TA) command for adjustingUE's uplink transmission timing, uplink transmission power information,power ramping information, UL grant for Msg3 transmission, a temporaryID, etc. In the conventional LTE/LTE-A system, RAR transmissiongenerally indicates that UE's RACH transmission is successful. Thus,upon receiving an RAR for a RACH preamble which has been transmitted ona specific cell/carrier, a UE performs uplink transmission based on thecorresponding RAR unless a radio link on the corresponding cell/carrieris disconnected or a RACH procedure is retriggered by a PDCCH order andthe like, instead of retransmitting the RACH preamble.

According to the present invention, an RAR may include network loadinformation in a specific beam direction. That is, by signaling whetherthere are large or small loads, an eNB may allow a UE who transmits acorresponding preamble to make RACH attempt to another beam. Ifreceiving an RAR indicating that there are large loads in the directionwhere the PRACH is transmitted, the UE performs a RACH procedure foranother beam direction on the corresponding cell/carrier even though theUE receives the RAR for the PRACH transmission corresponding to thecorresponding beam direction. The load information in an RAR may be usedas a beam switching command. The network may designate a separatepreamble for beam switching in an RAR message. When instructing beamswitching through an RAR, the eNB may inform preamble transmission powerfor another beam direction. If the RAR containing the beam switchinginstruction has no signaling related to transmission power, the UE mayconsider the PRACH transmission for another beam direction as PRACHretransmission and then perform the retransmission by ramping up thepower. In this case, the UE may select a beam direction with the bestquality among beam directions except the beam direction in which theload information is signaled and then transmit a PRACH preamble for theselected beam direction.

When an RAR for a PRACH configuration corresponding to a specific beamindex is transmitted/received, the RAR configuration for each PRACHconfiguration may be established independently or commonly. Aftertransmitting a PRACH, a UE may expect that an RAR will be transmittedwithin a specific window after k subframes or a specific time from thePRACH transmission time. In other words, the UE may expect that the RARfor a PRACH will be transmitted within a predetermined time period(i.e., time window) from the k-th subframe (or specific time) after theUE transmits the PRACH. For example, if a UE transmits a PRACH insubframe n, the UE may expect that an RAR for the PRACH will be receivedwithin a specific time period (i.e., RAR arrival window) from subframek. Here, the value of k and the RAR arrival window may besignaled/defined per PRACH configuration. In other words, these twovalues may be signaled/defined per beam index. Alternatively, the valueof k and the RAR arrival window may be signaled or defined commonly forall PRACH configurations. Although each beam direction has a differentPRACH resource, multiple RACHs belonging to the same beam-group mayshare the same RAR window. In this case, although PRACH preambles for aplurality of beam directions are transmitted on different time resourcesin an uplink time period corresponding to the downlink time periodrequired for the eNB to transmit SSs/PBCHs in all beam directions, theUE may expect that an RAR(s) for the PRACH preambles will be received onthe same time resource.

FIGS. 13A and 13B illustrate Random Access Response (RAR) messageformats according to the present invention.

Let's assume that a UE transmits preamble 1 on an RACH resource for beamindex A and (the same/a different) UE transmits preamble 2 on an RACHresource for beam index B. In this example, an RAR for preamble 1 and anRAR for preamble 2 can be transmitted in the same window.

Particularly, responses for multiple PRACHs may be transmitted as asingle RAR message. In other words, responses for PRACH transmission forvarious beam indices or SS blocks may be included in one RAR message. Inthis case, which PRACH response in an RAR message corresponds to whichbeam index may also be signaled. In particular, if one RACH resource isshared by a plurality of beam indices or SS block indices because an eNBhas low beam correspondence, responses for RACH preamble transmissionsassociated with the plurality of beam indices may be transmitted in oneRAR message. In this case, referring to FIG. 13A, beam indices forindividual PRACH responses are signaled in one RAR message.

Alternatively, an RAR may be transmitted per beam index. In this case,each RAR message carries a corresponding beam index. PRACH responses fora corresponding beam index may be transmitted in an RAR message. Forexample, referring to FIG. 13B, RAR 1 and RAR 2 for preamble 1 andpreamble 2, which are transmitted for beam direction A, are transmittedtogether with beam index A in one RAR message, and RAR 1 and RAR 2 forpreamble 1 and preamble 2, which are transmitted for beam direction B,are transmitted together with beam index B in another RAR message.

When N PRACH transmission time units corresponding to K beam indices aredeployed consecutively or uniformly, M RAR message reception time unitscorresponding to N PRACH signals may be deployed consecutively oruniformly. In this case, M may be equal to or less than N, and N may beequal to or less than K. According to this method, since RARs aresequentially arranged in PRACH transmission order, which RAR is forwhich beam may be automatically recognized. Assuming that a set of M RARtime units is an RAR window, a UE may perform RAR detection/receptionoperation as follows. The UE first performs PRACH signal transmissionthrough a PRACH time unit corresponding to its preferred beam index andsequentially checks beam indices included in a received RAR, forexample, starting at the first RAR time unit in the RAR window.

If a beam index included in the received RAR is equivalent to itspreferred beam index or the beam index corresponding to the PRACH timeunit transmitted by the UE and/or if a PRACH preamble ID included in theRAR is equivalent to that of the PRACH signal transmitted by the UE, theUE may perform subsequent operation (e.g., Msg3 transmission) accordingto the corresponding RAR and stop the RAR detection/reception operationin the corresponding RAR window.

If the beam index (and/or PRACH preamble ID) included in the RAR(s)received through all the M RAR time units in the RAR window is notequivalent to the PRACH signal transmitted by the UE, the UE mayconsider that the RAR reception fails and then perform subsequentoperation (e.g., PRACH signal retransmission, PRACH power ramping,and/or PRACH transmission count increment).

According to the present invention, the eNB can efficiently useresources by configuring/transmitting only the RAR for the actuallyreceived PRACH in the RAR window. For example, the eNB may transmit RARsin first some symbols/periods within the RAR window and transmit DL/ULdata/control channels in the remaining symbols.

* PRACH Retransmission, RACH Resource Selection, and/or Power Control(Method 1)

The random access procedure of the 3GPP LTE system can be summarized asfollows.

From the physical layer perspective, the L1 random access procedureencompasses the transmission of random access preamble and random accessresponse. The remaining messages are scheduled for transmission by thehigher layer on the shared data channel and are not considered part ofthe L1 random access procedure. A random access channel occupies 6resource blocks in a subframe or set of consecutive subframes reservedfor random access preamble transmissions. The eNB is not prohibited fromscheduling data in the resource blocks reserved for random accesschannel preamble transmission. The following steps are required for theL1 random access procedure.

Layer 1 procedure is triggered upon request of a preamble transmissionby higher layers.

A preamble index, a target preamble received power(PREAMBLE_RECEIVED_TARGET_POWER), a corresponding RA-RNTI and a PRACHresource are indicated by higher layers as part of the request.

A preamble transmission power PPRACH is determined asP_(PRACH)=min{P_(CMAX,c)(i),PREAMBLE_RECEIVED_TARGET_POWER+PL_(c)}_[dBm], where P_(CMAX,c)(i) is theconfigured UE transmit power defined in 3GPP TS 36.101 for subframe i ofserving cell c and PL_(c) is the downlink path loss estimate calculatedin the UE for serving cell c.

A preamble sequence is selected from the preamble sequence set using thepreamble index.

A single preamble is transmitted using the selected preamble sequencewith transmission power PPRACH on the indicated PRACH resource.

Detection of a PDCCH with the indicated RA-RNTI is attempted during awindow controlled by higher layers (see 3GPP TS 36.321, section 5.1.4).The higher layers parse the transport block and indicate the 20-bituplink grant to the physical layer.

Meanwhile, the random access procedure at the medium access control(MAC) layer can be performed as follows.

set PREAMBLE_RECEIVED_TARGET_POWER to‘preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep’;

If the UE is a bandwidth limited (BL) UE or a UE in enhanced coverage:

instruct the physical layer to transmit a preamble with the number ofrepetitions required for preamble transmission corresponding to theselected preamble group (i.e., numRepentionPerPreambleAttempt) using theselected PRACH corresponding to the selected coverage enhancelement (CE)level, corresponding RA-RNTI, preamble index, andPREAMBLE_RECEIVED_TARGET_POWER.

else:

instruct the physical layer to transmit a preamble using the selectedPRACH, corresponding RA-RNTI, preamble index andPREAMBLE_RECEIVED_TARGET_POWER.

In the LTE/LTE-A system, information on UL transmission power for PRACHpreamble transmission is included in an RACH configuration andtransmitted to a UE. For example, UE-common random access parameterssuch as preambleInitialReceivedTargetPower, powerRampingStep,preambleTransMax, etc. are transmitted to the UE through an RRC signal(see PRACH-Config of TS 36.331). PREAMBLE_TRANSMISSION_COUNTER startsfrom 1 and increases by 1 whenever preamble transmission is attempted.The maximum number of times that preamble transmission can be performedis defined as preambleTransMax, and the preamble transmission can berepeatedly performed no more than preambleTransMax. For example, ifPREAMBLE_TRANSMISSION_COUNTER=preambleTransMax+1, the MAC layer informshigher layers of a random access problem and considers that the randomaccess procedure is unsuccessfully completed. DELTA_PREAMBLE has thefollowing predefined values (see Table 7.6-1 of 3GPP TS 36.321).

TABLE 2 Preamble Format DELTA_PREAMBLE value 0  0 dB 1  0 dB 2 −3 dB 3−3 dB 4  8 dB

In Table 2, the preamble formats are given by prach-ConfigIndex (seePRACH-ConfigIndex of TS 36.331).

As described above, in the LTE/LTE-A system, the PRACH preambletransmission power is determined according to equation (1) below.P _(PRACH)=min{P _(CMAX,c)(i), PREAMBLE_RECEIVED_TARGET_POWER+PL_(c)}_[dBm].  Equation (1)

In a multi-beam environment, a plurality of SS blocks can be defined.Specifically, a plurality of SS blocks are defined, and each of the SSblocks can be transmitted in its unique DL transmission beam direction.In addition, a RACH resource may be configured per SS block. A UE mayreceive signals/channels in the SS blocks and select an SS block indexwith the best reception quality. In addition, the UE may select an RACHresource associated with the corresponding SS block index and transmit aPRACH preamble on the selected resource. In this case, the UE may selectone or more SS block indices and attempt PRACH transmission on the RACHresource associated with each SS block.

If the UE fails to receive an RAR within a corresponding RAR window, theUE attempts the PRACH transmission again and repeat the above-describedprocesses. It is called PRACH retransmission. Whenever performing thePRACH retransmission, the UE ramps up the PRACH transmission power by acertain degree. That is, the UE performs the retransmission according tothe number of allowed PRACH retransmission rounds. Whenever performingthe retransmission, the UE ramps up the power, but the ramped-up powercannot exceed the maximum transmission power. The number of times thatPRACH retransmission can be performed is reflected in the variablePREAMBLE_TRANSMISSION_COUNTER of Equation (2) below, and the amount oframped-up power is reflected in powerRampingStep. Eventually, the PRACHpreamble transmission power of the LTE system is determined according toEquation (1) above and Equation (2) below.PREAMBLE_RECEIVED_TARGET_POWER=preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep.  Equation(2)

Before describing the transmission power for the PRACH preambleretransmission in the new RAT system, the PRACH retransmission needs tobe defined. In particular, in a multi-beam environment, PRACH preamblerepetition or beam sweeping may be considered according to TX/RXreciprocal capability of a TRP or UE. The TX/RX reciprocal capabilitycould be referred to as TX/RX beam correspondence at the TRP and a UE.If the TRP and UE cannot maintain the TX/RX reciprocal capability in themulti-beam environment, the UE may be unable to transmit an uplinksignal in the beam direction where the UE receives a downlink signal.This is because the UL optimal path may be different from the DL optimalpath. If the TRP can determine a TRP RX beam for corresponding uplinkreception based on UE's downlink measurements for one or more TX beamsof the TRP and/or if the TRP can determine a TRP TX beam forcorresponding downlink transmission based on TRP's uplink measurementsfor one or more RX beams of the TRP, the TX/RX beam correspondence atthe TRP can be hold. Meanwhile, if the UE can determine a UE RX beam forcorresponding uplink transmission based on UE's downlink measurementsfor one or more RX beams of the UE and/or if the UE can determine a UETX beam for corresponding downlink reception based on an indication fromthe TRP, which is made based on uplink measurements for one or more TXbeams of the UE, the TX/RX beam correspondence at the UE can be hold.

FIG. 14 illustrates PRACH transmission according to the presentinvention. Although FIG. 14 shows that PRACH resources for a set of beamdirections applied to a cell/carrier are consecutive in the time domain,the PRACH resources can be arranged nonconsecutively. In addition,although FIG. 14 shows the PRACH resources for the set of beamdirections available on the cell/carrier are the same in the frequencydomain, different frequency resources may be configured.

An RACH resource can be defined per beam direction in which an SS istransmitted, or per SS block. At this time, the corresponding RACHresource may be sub-divided into RACH basic units. Here, the RACH basicunit can be defined as a time-frequency resource used for transmittingone PRACH preamble. For example, referring to FIG. 14 , one PRACHresource configured for one beam direction or SS block can be segmentedinto two RACH basic units. Although FIG. 14 shows that one PRACHresource is segmented into two RACH basic units, the PRACH resource maybe segmented into three or more RACH basic units.

One-time PRACH attempt may mean that a PRACH preamble is transmitted ona RACH resource defined per SS block. In addition, although preamblerepetition or beam sweeping is performed on the corresponding RACHresource, it may be considered as one-time PRACH attempt. For example,even if a UE transmits RACH preambles on the same RACH resource bychanging a beam direction according to RACH basic units, it may beconsidered as one-time PRACH attempt. In other words, if a plurality ofPRACH preambles are transmitted using different RACH basic units on thesame RACH resource, it is not considered as retransmission. For example,referring to FIG. 14 , if a UE transmits RACH preambles on RACH basicunits of a PRACH preamble resource for SS block 1 (in differentdirections), this may be considered as one-time RACH transmissionattempt. Thus, when preambles are repeatedly transmitted on the sameRACH resource or when the beam sweeping where transmission is performedby changing a beam direction is applied, transmission power may notincrease per preamble transmission.

However, even in the case of RACH resources associated with the samebeam or SS block, if an RACH preamble is transmitted on a next RACHresource (that is, when a UE stands by during an RAR window for RARreception and then transmits an RACH preamble on an RACH resource afterthe RAR window) or if a PRACH preamble is transmitted on a PRACHresource associated with another beam/SS block, it is considered asretransmission. For example, referring to FIG. 14 , if a UE does notreceive an RAR for a RACH preamble transmitted on a PRACH preambleresource associated with SS block 1 or receives an RAR indicating beamswitching, the UE may transmit a RACH preamble on a next PRACH preambleresource for SS block 1 or a PRACH preamble resource associated withanother SS block. At this time, the RACH preamble transmission isconsidered different from the previous RACH preamble transmission. Inthis case, “PREAMBLE_TRANSMISSION_COUNTER” of Equation (2), whichindicates how many times PRACH preamble retransmission is performed,increases. In other words, if the RACH preamble transmission using RACHresources for the same beam or SS block occurs in different PRACHopportunities, or if RACH preamble transmission uses RACH resourcesassociated with different beams or SS blocks, a UE increase“PREAMBLE_TRANSMISSION_COUNTER”.

The amount of ramped-up power may vary per beam index. The amount ofpower ramped up by a UE may be configured/signaled independently betweenbeam indices, and the power increase for retransmission may be computedper beam index. However, the calculation ofPREAMBLE_TRANSMISSION_COUNTER is performed by collecting all RACHresources even if different beam directions, i.e., different RACHresources are used. For example, PRACH transmission power is obtained bycalculating how many times each UE performs PRACH preamble transmissioninstead of calculating how many times the PRACH preamble transmission isperformed per RACH resource. This is because since RACH transmissionfails even though power is ramped up by attempting the RACH transmissionfor beams with good quality, the delay of the RACH transmissionincreases if the power is set to the default value on the next RACHresource. However, since each beam direction, i.e., each SS block has adifferent RS reception level, path loss values may vary. Thus, when theUE performs the PRACH preamble retransmission, the UE compensates forthe path loss of the PRACH preamble transmission power in each RACHresource used for the PRACH preamble transmission. For example, assumingthat RACH resource j is associated with beam direction or SS block indexj, PRACH transmission power PPRACH, J on RACH resource j can be definedby Equations (3) and (4).P _(PRACH,j)=min{P _(CMAX,c)(i), PREAMBLE_RECEIVED_TARGET_POWER+PL_(c,beam(j))}_[dBm].  Equation (3)

where P_(CMAX,c)(i) is the configured UE transmit power for subframe iof serving cell c and PL_(c,beam(j)) is the downlink path loss estimatecalculated in the UE for beam direction j (or SS block index j) ofserving cell c.PREAMBLE_RECEIVED_TARGET_POWER=preambleInitialReceivedTargetPower+DELTAPREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep(j).  Equation(4)

where, if powerRampingStep is configured RACH resource common,powerRampingStep(j) is powerRampingStep.

If preambleInitialReceivedTargetPower and preamble formats areconfigured per RACH resource (i.e., per beam direction or SS blockindex), preambleInitialReceivedTargetPower and DELTA_PREAMBLE can berespectively changed to preambleInitialReceivedTargetPower(j) andDELTA_PREAMBLE(j) in Equation 4.

When a UE attempts the PRACH preamble transmission using a plurality ofRACH resources, the network can configure and signal the number of RACHresources that can be used by the UE. In addition, if the PRACH preambletransmission is allowed on two RACH resources, the network may configureoffsets for received signal levels of the best and second best beams andthen transmit the offsets by including them in a RACH configuration.When there are more than two RACH resources, offsets for received signallevels of the best, second best, third best beams are signaled. This maymean that the UE can attempt the PRACH preamble transmission withrespect to beams only within the corresponding offsets as wells as thebest beam.

* PRACH Retransmission, RACH Resource Selection Method, and/or PowerControl

When a plurality of SS blocks are transmitted, a plurality of RACHresources can be allocated for a UE. Hereinafter, a method by which theUE selects a RACH resource for a random access procedure among theplurality of RACH resources will be described. In particular, the methodfor selecting a RACH resource for a random access procedure, i.e., RACHprocedure will be described relating to a PRACH retransmission method.

>Alt a. Best Beam First—Based on Received Signal Level of SS Block

According to Alt a, when a UE performs PRACH transmission, if aplurality of SS blocks are received above a certain level, in otherwords, if the UE has multiple preferred beam indices, the UE firsttransmits a PRACH for an SS block with the highest received signalstrength (i.e., the best beam index). If the UE does not receive an RARfor the corresponding PRACH within a given RAR window, the UEretransmits the PRACH for the corresponding beam index. In other words,the UE preferentially transmits a PRACH preamble on a RACH resourceassociated with the SS block index with the highest received signalstrength, and when the retransmission is required, the UE preferentiallyattempts to transmit a PRACH preamble for the SS block index with thehighest received signal strength. For convenience of description, it isassumed that a UE prefers three beam indices: beam indices a, b and c.In addition, it is also assumed that among the preferred beams, beamindex a is the best beam and beam index c is the beam with the lowestquality. The numbers of retransmission rounds for beam indices a, b andc are denoted by Ra, Rb and Rc, respectively. If the UE does not receivean RAR for a PRACH for the best beam index after performingretransmission for the best beam index Ra times (where Ra≥1), the UE mayattempt retransmission for the beam index with the second best qualityRb times (where Rb≥1). If the UE still does not receive any RAR in spiteof attempting the PRACH retransmission Rb times, the UE may attemptretransmission for the beam index with the third best quality Rc times(where Rc≥1). In other words, the number of PRACH preambleretransmission rounds for each SS block may be independently defined orconfigured. The UE preferentially attempts to transmit a PRACH preamblefor the SS block index with the highest received signal strength. And,if necessary, the UE attempts retransmission for the SS block index withthe highest received signal strength as many times as possible withinthe maximum number of retransmission rounds. If the UE still does notreceive any RAR from the network despite the retransmission attempts orif the UE does not completes the RACH procedure due to contentionresolution failure even though the UE receive a RAR, the UE may attemptthe PRACH transmission for the SS block with the second best receivedquality.

Whenever the UE performs PRACH retransmission, the UE ramps up power bya value (delta) signaled to the UE. That is, whenever a PRACH for beamindex a is transmitted, power is ramped up. The PRACH for beam index ais retransmitted Ra times, and the power is ramped up everyretransmission until the maximum power is reached. When the maximumpower is reached, the retransmission for beam index a is performed withthe maximum power. If the UE does not receive an RAR even though the UEretransmits the PRACH for beam index a Ra times, the UE transmits aPRACH for beam index b on a PRACH resource associated with beam index b.When the UE transmits the PRACH for the beam index b, the UE shouldreset or initialize the transmission power. That is, the maximum powerused for the PRACH transmission for beam index a is not used. As if thePRACH was transmitted at the first time, the PRACH for beam index b istransmitted at the power used for the initial transmission. Thereafter,if the UE does not receive an RAR, the power is ramped up. Similarly, ifthe UE does not receive an RAR in spite of retransmitting the PRACH forbeam index b Rb times, the UE transmits a PRACH for beam index c, whichhas the next best quality. When the UE attempts the PRACH preambletransmission by changing the current RACH resource, the UE may calculatethe initial transmission power with respect to the received signalstrength (e.g., RSRP) of the SS block associated with the correspondingRACH resource similar to the power control method described in Method 1.That is, when the UE performs the PRACH preamble retransmission, thePRACH preamble transmission power may compensate for the path loss ineach RACH resource or SS block index used for the PRACH preambletransmission.

When the UE fails to receive any RAR for RRACHs, that is, when the UEdoses not receive an RAR for its all preferred beams, the UE may reportthis fact to higher layers and then perform cell reselection operation.For example, unlike the power control method described in Method 1, inthe case of RACH retransmission for different beam indices, the UE mayattempt power ramping per beam index using a separate power rampingcounter per beam index. That is, when the UE performs RACH preambleretransmission for a different beam index (i.e., SS block index),retransmission rounds are not counted unlike Method 1 whereretransmission rounds are counted regardless of beam indices (i.e., SSblock indices).

>Alt b. Beam Index Round Robin

According to Alt b, when a UE performs PRACH transmission, if aplurality of SS blocks are received above a certain signal strengthlevel, in other words, if the UE has multiple preferred beam indices,the UE first transmits a PRACH for an SS block with the highest receivedsignal strength (i.e., the best beam index). For example, when the UEhas multiple preferred beam indices, the UE first transmits a PRACH forthe best beam index. Thereafter, if the UE does not receive an RAR forthe corresponding PRACH within a given RAR window, the UE transmits aPRACH for the beam index with the second best reception quality. If theUE does not receive an RAR for the corresponding PRACH in the RARwindow, the UE transmits a PRACH for the beam index with the third bestreception quality. In other words, when a UE has a plurality of SS blockindices, each of which having received signal quality above a certainlevel, the UE may perform PRACH preamble retransmission by sequentiallyselecting RACH preamble transmission for the plurality of SS blockindices according to the SS block reception quality. However, if a UEperforms PRACH preamble transmission with respect to a number of RACHresources, it may degrade the system performance due to the ping-pongeffect. Hence, restrictions may be imposed on the number of PRACHresources where a UE can attempt the PRACH preamble transmission and thereception quality range of associated SS blocks. For example, a networkmay signal the maximum number of SS blocks where a UE makes RACHattempts or the number of RACH resources, an offset value from thereceived signal strength of the best SS block, etc. via a PRACHconfiguration. The offset may be used to allow the RACH preambletransmission only for SS block indices within a certain range from thereceived signal strength of the best SS block.

When a UE does not receive an RAR after transmitting a PRACH for aspecific beam index, the UE may transmit a PRACH based on the secondbest quality. In this case, the UE may consider it as PRACHretransmission and then ramp up the power. However, since although it isconsidered as the PRACH retransmission from the perspective of the UE,the corresponding transmission is PRACH transmission for another beamindex, it is preferred not to ramp up the power. Thus, when the UEtransmits individual PRACHs for a set of the preferred beam indices, ifthe UE transmits the PRACH for the specific beam index for the firsttime, the UE does not ramp up the power. In addition, if the UE does notreceive an RAR after attempting the PRACH transmission one time for allbeam indices, the UE transmits the PRACH for the best beam index againby ramping up the power. In performing power ramping-up, the UE mayattempt the power ramping-up per beam index by separately using a powerramping counter per SS block index (or beam index) as mentioned in Alta. However, since each beam direction, i.e., each SS block has adifferent reception level, path loss values may vary. Thus, when the UEperforms the PRACH preamble retransmission, the UE may compensate forthe path loss in each RACH resource or SS block index used for thecorresponding PRACH preamble transmission using the corresponding PRACHpreamble transmission power. For convenience of description, it isassumed that a UE prefers three beam indices: beam indices a, b and c.In addition, it is also assumed that among the preferred beams, beamindex a is the best beam and beam index c is the beam with the lowestquality. The numbers of retransmission rounds for beam indices a, b andc are denoted by Ra, Rb and Rc, respectively. If the UE does not receivean RAR after transmitting a PRACH for beam index a, the UE transmits aPRACH for beam index b. If the UE receive no RAR for the PRACH for beamindex b, the UE transmits a PRACH for beam index c. If the UE does notreceive any RAR for all the beam indices, the UE transmits the PRACH forbeam index a again by ramping up the power. If the UE still does notreceive an RAR for the PRACH for beam index a, the UE transmits thePRACH for beam index b, which is the next beam index, with the ramped uppower, that is the power that is previously used for transmitting thePRACH for beam index a. Similarly, if the UE does not receive an RAR forthe PRACH for the beam index b, the UE transmits the PRACH for beamindex c with the same power. That is, the UE can perform the PRACHretransmission for each beam index by ramping up the power as describedabove. As another method for setting PRACH preamble transmission power,when a UE performs retransmission on the same RACH resource, i.e., theRACH resource associated with the same beam or SS block, the UE may rampup the power. And, when the UE changes the RACH resource, the UE may usethe previous power as it is without resetting or initializing the PRACHtransmission power. That is, when the UE performs the retransmission onthe same RACH resource, the UE may ramp up the power. On the contrary,when the UE performs the retransmission by changing the RACH resource,the UE may maintain the previous PRACH preamble transmission power.

>Alt c. Multiple PRACH Preamble Transmission Scheme

According to Alt c, if a UE receives a plurality of SS blocks above acertain signal strength level, in other words, if the UE has multiplepreferred beam indices, the UE can transmit PRACH preambles for theplurality of SS blocks. That is, if the UE does not receive an RAR forany one of the beam indices after transmitting respective PRACHs for theplurality of beam indices, the UE may attempt the PRACH transmissionagain. For example, after respectively transmitting PRACHs for beamindices a, b and c, the UE may wait for RARs for the transmitted PRACHsin the same window or overlapping windows. That is, even though the UEdoes not receive an RAR for a specific beam index, the UE may transmit aPRACH for another beam index. When the UE receives no RAR for beamindices a, b, and c, the UE may retransmits the PRACHs for beam indicesa, b and c. In this case, the UE ramps up and transmits the PRACH foreach of the beam indices.

The UE may transmit a plurality of PRACHs without waiting for RARreception. However, if PRACHs are for the same beam index, the UE cannotperform the PRACH transmission until receiving an RAR, that is, beforereaching a new RAR transmission window. In other words, only when PRACHsare for different beam indices, the UE can perform the PRACHtransmission without waiting for the RAR reception.

Although it is described that a UE can attempt PRACH transmission byrotating beam indices in a round robin manner, the UE may determine thePRACH transmission order and the number of PRACH transmission roundswhen performing the round robin rotation for the beam indices.Preferably, the best beam index starts first. When a specific beam indexhas channel quality above a certain threshold, if the reception qualityof the specific beam index is significantly different from that ofanother beam index, the UE may attempt PRACH transmission for the beamindex with the better quality more frequently. For example, the UE mayattempt RACH transmission in the form of [a, b, a, b, c, a, b, a, b, c,. . . ] rather than [a, b, c, a, b, c, . . . ]. In this case, the powerramping principle can be established such that in the case ofretransmission for the same beam index, power is ramped up, and fordifferent beam indices, power is ramped up by the number ofretransmission rounds for a corresponding beam index.

Among candidate resources where power ramping-up will be applied, thefirst available PRACH resource is the first available resource amongavailable RACH resources or the resource capable of reducing the totaldelay maximally. As another method, the RACH resource corresponding tothe beam with a largest amount of resources may be selected as the firstavailable PRACH resource, or an RACH resource may be selected as thefirst available PRACH resource by considering loads and the like asdescribed above.

* Maximum PRACH Transmission Number

The maximum number of times that a UE can perform PRACH (re)transmissionshould be defined. When a UE transmits PRACHs for a plurality of beamindices, the maximum retransmission number can be mainly definedaccording to the following two method.

>Method 1. The maximum (re)transmission number R can be designated perUE. In this case, the condition of R=Ra+Rb+Rc may be established in theabove-described example. That is, the maximum PRACH retransmissionnumber is defined and signaled per UE, and this value may be the sum ofthe maximum retransmission number per beam index. The maximumretransmission number per beam index may be the same or different. AneNB or UE may increase the number of PRACH retransmission rounds for thebest beam index. For example, in the above example, Ra>Rb>Rc.Alternatively, Ra=Rb=Rc. The both cases should satisfy the condition ofRa+Rb+Rc=R.

>Method 2. The maximum retransmission number R can be designated perbeam index. In this case, the condition of R=Ra=Rb=Rc may be establishedin the above-described example. That is, when the number of times that aUE can perform PRACH retransmission per beam index is defined andsignaled and when the same number of PRACH retransmission rounds is setper beam index, it can be signaled as a single value. However,considering the fact that a PRACH configuration is set per beam index,if the maximum retransmission number is set per beam index, it isdesirable that the corresponding value is signaled per beam index. Inthis case, Ra≠Rb≠Rc, and thus Ra, Rb, and Rc are respectively signaled.A UE performs PRACH retransmission as many times as specified by themaximum retransmission number for a specific beam index and thenattempts PRACH transmission for another beam index. Even if the maximumretransmission number is defined per beam index, the UE may autonomouslydetermine the maximum retransmission number per beam index. That is,even if the maximum retransmission number of beam index a is set to Ra,the UE may perform PRACH retransmission Rx times (where Rx<Ra).

When a UE transmits PRACHs for a plurality of beam indices, if thenumber of desired beam indices is too high, the corresponding UE shouldperform PRACH transmission too many times, and thus the UE may waste toomany resources. Therefore, the maximum PRACH transmission number per UE,Rmax may be signaled. In this case, Rmax may satisfy the condition ofRmax=<Ra+Rb+Rc.

* PRACH Resource Configuration Depending on UE Coverage

Configuring PRACH resources per beam may cause resource waste. When a UEis relatively far away from an eNB, the UE may require beamforming. Forthis UE, it is desirable to configure PRACH resources per beam index,but in the case of UEs located at the cell center, the eNB does not needto perform RX beamforming. Thus, according to UE coverage classes, PRACHresources can be differently configured. A UE may transmit a PRACH oncommon PRACH resources, and the eNB may be configured not to perform theRX beamforming on the corresponding resources. Since the correspondingresource region is not limited to a specific beam direction, it ispossible to avoid resource waste. If UEs are located at the cell center,the UEs may have almost no propagation loss or blockage. Such UEs, i.e.,UEs with excellent channel states may be configured to perform PRACHtransmission on resources where omni-direction reception is possible.For the corresponding UEs, common PRACH resources can be separatelyallocated. The UEs may repeatedly transmit PRACHs on the common PRACHresources regardless of beam indices.

If a UE is relatively far away from the eNB or requires the beamforming,the UE may transmit a PRACH per beam index using beam-index-specificPRACH resources according to the aforementioned methods.

Referring to the aforementioned sequential PRACH transmission methodagain, a UE transmits a PRACH for the best beam index. And, if no RAR istransmitted within a given window, the UE attempts the PRACHtransmission for the corresponding beam index again. The UE may attemptthe retransmission x times (where x≥1). If the UE fails to receive anRAR after performing the retransmission x times, the UE transmits aPRACH for the beam index with the second best quality and then waits toreceive an RAR within the given window. Similarly, if the UE receives noRAR for the corresponding PRACH, the UE may perform PRACH retransmissionfor the corresponding beam index y times (where y≥1). If the UE stilldoes not receive any RAR after performing the PRACH retransmission ytimes, the UE may attempt PRACH transmission for the beam index with thethird best quality. The number of times that the UE attempts the PRACHtransmission may be the same per beam index (i.e., x=y). Alternatively,the number of times that the UE attempts the PRACH transmission may bedifferent per beam index, and in this case, if a specific beam index hasbetter reception quality, the number of PRACH attempts for the specificbeam index may increase.

* RACH Message 3 Transmission

After transmitting a PRACH for a specific beam index, a UE may receivean RAR for the corresponding PRACH. In this case, the UE may transmitits C-RNTI and a set of preferred beam indices (e.g., SS block indices)if possible. After receiving the report about the beam indices, an eNBmay select a specific beam index from the corresponding beam index setand transmit the selected beam index when scheduling the correspondingUE.

For example, PRACH Message 3 may contain the index of one preferredbeam, which is obtained by a UE through downlink beam RS measurement orN beam indices (where N is a random integer greater than 1) and thereceived signal strength for the corresponding beam indices (e.g.,RSRSP).

* RACH Message 4 Transmission

Upon receiving RACH Message 3 from a UE, an eNB transmits RACH Message 4to the corresponding UE. Generally, RACH Message 4 is transmitted forcontention resolution. However, according to the present invention,based on best N beam indices (beam information) reported by a UE, an eNBsignals a beam index to be used for data transmission to thecorresponding UE in RACH Message 4. In this case, the eNB can signal oneor more beam indices. By doing so, the UE expect that its PDCCH/PDSCHwill be transmitted in the direction corresponding to the signaled beamindex and may perform RX beamforming in the corresponding direction.Similarly, when the UE transmits a PUSCH, the UE may perform uplinktransmission such as PUSCH/PUCCH in the direction corresponding to thesignaled beam index.

* Beam Index Update

A UE may report its preferred beam indices as follows. The followingmethods can also be used when UE's preferred beam directions arechanged/added.

>Method 1: When obtaining PRACH resource information per beam index, aUE transmits a PRACH on a PRACH resource per beam index. By doing so,the UE may informs an eNB that the channel quality of the correspondingbeam index is good.

>Method 2: A UE may perform RSRP measurement by measuring an RS per beamindex. In this case, a Beam RS (BRS), which is transmitted throughwideband or a PBCH-RS, which is used for PBCH demodulation, may be usedas a reference RS for performing the RSRP measurement. Alternatively,RSRQ measurement may be performed with reference to the received signalstrength of PSS/SSS.

FIG. 11 is a block diagram illustrating elements of a transmittingdevice 10 and a receiving device 20 for implementing the presentinvention.

The transmitting device 10 and the receiving device 20 respectivelyinclude Radio Frequency (RF) units 13 and 23 capable of transmitting andreceiving radio signals carrying information, data, signals, and/ormessages, memories 12 and 22 for storing information related tocommunication in a wireless communication system, and processors 11 and21 operationally connected to elements such as the RF units 13 and 23and the memories 12 and 22 to control the elements and configured tocontrol the memories 12 and 22 and/or the RF units 13 and 23 so that acorresponding device may perform at least one of the above-describedexamples of the present invention.

The memories 12 and 22 may store programs for processing and controllingthe processors 11 and 21 and may temporarily store input/outputinformation. The memories 12 and 22 may be used as buffers.

The processors 11 and 21 generally control the overall operation ofvarious modules in the transmitting device and the receiving device.Especially, the processors 11 and 21 may perform various controlfunctions to implement the present invention. The processors 11 and 21may be referred to as controllers, microcontrollers, microprocessors, ormicrocomputers. The processors 11 and 21 may be implemented by hardware,firmware, software, or a combination thereof. In a hardwareconfiguration, application specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), or field programmable gate arrays(FPGAs) may be included in the processors 11 and 21. Meanwhile, if thepresent invention is implemented using firmware or software, thefirmware or software may be configured to include modules, procedures,functions, etc. performing the functions or operations of the presentinvention. Firmware or software configured to perform the presentinvention may be included in the processors 11 and 21 or stored in thememories 12 and 22 so as to be driven by the processors 11 and 21.

The processor 11 of the transmitting device 10 performs predeterminedcoding and modulation for a signal and/or data scheduled to betransmitted to the outside by the processor 11 or a scheduler connectedwith the processor 11, and then transfers the coded and modulated datato the RF unit 13. For example, the processor 11 converts a data streamto be transmitted into K layers through demultiplexing, channel coding,scrambling, and modulation. The coded data stream is also referred to asa codeword and is equivalent to a transport block which is a data blockprovided by a MAC layer. One transport block (TB) is coded into onecodeword and each codeword is transmitted to the receiving device in theform of one or more layers. For frequency up-conversion, the RF unit 13may include an oscillator. The RF unit 13 may include N_(t) (where N_(t)is a positive integer) transmit antennas.

A signal processing process of the receiving device 20 is the reverse ofthe signal processing process of the transmitting device 10. Undercontrol of the processor 21, the RF unit 23 of the receiving device 20receives radio signals transmitted by the transmitting device 10. The RFunit 23 may include N_(r) (where N_(r) is a positive integer) receiveantennas and frequency down-converts each signal received throughreceive antennas into a baseband signal. The processor 21 decodes anddemodulates the radio signals received through the receive antennas andrestores data that the transmitting device 10 intended to transmit.

The RF units 13 and 23 include one or more antennas. An antenna performsa function for transmitting signals processed by the RF units 13 and 23to the exterior or receiving radio signals from the exterior to transferthe radio signals to the RF units 13 and 23. The antenna may also becalled an antenna port. Each antenna may correspond to one physicalantenna or may be configured by a combination of more than one physicalantenna element. The signal transmitted from each antenna cannot befurther deconstructed by the receiving device 20. An RS transmittedthrough a corresponding antenna defines an antenna from the view pointof the receiving device 20 and enables the receiving device 20 to derivechannel estimation for the antenna, irrespective of whether the channelrepresents a single radio channel from one physical antenna or acomposite channel from a plurality of physical antenna elementsincluding the antenna. That is, an antenna is defined such that achannel carrying a symbol of the antenna can be obtained from a channelcarrying another symbol of the same antenna. An RF unit supporting aMIMO function of transmitting and receiving data using a plurality ofantennas may be connected to two or more antennas.

In the examples of the present invention, a UE operates as thetransmitting device 10 in UL and as the receiving device 20 in DL. Inthe examples of the present invention, an eNB operates as the receivingdevice 20 in UL and as the transmitting device 10 in DL. Hereinafter, aprocessor, an RF unit, and a memory included in the UE will be referredto as a UE processor, a UE RF unit, and a UE memory, respectively, and aprocessor, an RF unit, and a memory included in the eNB will be referredto as an eNB processor, an eNB RF unit, and an eNB memory, respectively.

According to the present invention, the eNB processor may control theeNB RF unit to transmit a synchronization signal, a broadcast signal,and system information in accordance with any one of the proposals ofthe present invention. In addition, the eNB processor may control theeNB RF unit to receive a RACH from the UE in accordance with any one ofthe proposals of the present invention. Moreover, the eNB processor maycontrol the eNB RF unit to transmit PDCCH/PDSCH in accordance with theproposals of the present invention. Further, the eNB processor maycontrol the eNB RF unit to receive PUSCH/PUCCH in accordance with theproposals of the present invention.

According to the present invention, the UE processor may control the UERF unit to receive a synchronization signal, a broadcast signal, andsystem information in accordance with any one of the proposals of thepresent invention. In addition, the UE processor may control the UE RFunit to transmit a RACH in accordance with any one of the proposals ofthe present invention. Moreover, the UE processor may control the UE RFunit to receive PDCCH/PDSCH in accordance with the proposals of thepresent invention. Further, the UE processor may control the UE RF unitto transmit PUSCH/PUCCH in accordance with the proposals of the presentinvention.

As described above, the detailed description of the preferred examplesof the present invention has been given to enable those skilled in theart to implement and practice the invention. Although the invention hasbeen described with reference to examples, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific examples described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

The examples of the present invention are applicable to a BS, a UE, orother devices in a wireless communication system.

What is claimed is:
 1. A communication device comprising: at least onetransceiver; at least one processor; and at least one computer memorystoring computer-readable instructions that cause the at least oneprocessor to perform operations comprising: receiving a synchronizationsignal among a plurality of synchronization signals transmitted from acell; receiving configuration information regarding a plurality ofrandom access channel resources for the cell; and transmitting a randomaccess preamble to the cell based on receiving the synchronizationsignal, wherein each of the plurality of synchronization signals isrelated to a different downlink beam among a plurality of downlink beamsfor the cell, wherein each of the plurality of random access channelresources is related to at least one of the plurality of synchronizationsignals, and wherein the random access preamble is transmitted on arandom access channel resource, among the plurality of random accesschannel resources, which is related to the synchronization signal. 2.The communication device according to claim 1, wherein the configurationinformation is included in system information.
 3. The communicationdevice according to claim 1, wherein the plurality of synchronizationsignals are distinguished by different indices other than a frame index,a subframe index and a symbol index.
 4. The communication deviceaccording to claim 3, wherein the operations further comprise: receivinga random access response associated with the random access preamble; andperforming downlink channel reception or uplink channel transmissionbased on an index informed by the random access response among thedifferent indices.
 5. A method of transmitting a random access preambleby a communication device, the method comprising: receiving asynchronization signal among a plurality of synchronization signalstransmitted from a cell; receiving configuration information regarding aplurality of random access channel resources for the cell; andtransmitting the random access preamble to the cell based on receivingthe synchronization signal, wherein each of the plurality ofsynchronization signals is related to a different downlink beam among aplurality of downlink beams for the cell, wherein each of the pluralityof random access channel resources is related to at least one of theplurality of synchronization signals, and wherein the random accesspreamble is transmitted on a random access channel resource, among theplurality of random access channel resources, which is related to thesynchronization signal.
 6. The method according to claim 5, wherein theconfiguration information is included in system information.
 7. Themethod according to claim 5, wherein the plurality of synchronizationsignals are distinguished by different indices other than a frame index,a subframe index and a symbol index.
 8. The method according to claim 7,further comprising: receiving a random access response associated withthe random access preamble; and performing downlink channel reception oruplink channel transmission based on an index informed by the randomaccess response among the different indices.
 9. A base stationcomprising: at least one transceiver; at least one processor; and atleast one computer memory storing computer-readable instructions thatcause the at least one processor to perform operations comprising:transmitting a plurality of synchronization signals to a cell;transmitting configuration information regarding a plurality of randomaccess channel resources for the cell; and receiving a random accesspreamble from a communication device, wherein each of the plurality ofsynchronization signals is related to a different downlink beam among aplurality of downlink beams for the cell, wherein each of the pluralityof random access channel resources is related to at least one of theplurality of synchronization signals, and wherein the random accesspreamble is received on one among the plurality of random access channelresources.
 10. The base station according to claim 9, wherein theconfiguration information is included in system information.
 11. Thebase station according to claim 9, wherein the plurality ofsynchronization signals are distinguished by different indices, otherthan a frame index, a subframe index and a symbol index.
 12. The basestation according to claim 11, wherein the operations further comprise:transmitting a random access response including index informationregarding one among the different indices based on receiving the randomaccess preamble; and performing downlink channel transmission or uplinkchannel reception based on the index information.
 13. A method ofreceiving a random access preamble by a base station, the methodcomprising: transmitting a plurality of synchronization signals to acell; transmitting configuration information regarding a plurality ofrandom access channel resources for the cell; and receiving the randomaccess preamble from a communication device, wherein each of theplurality of synchronization signals is related to a different downlinkbeam among a plurality of downlink beams for the cell, wherein each ofthe plurality of random access channel resources is related to at leastone of the plurality of synchronization signals, and wherein the randomaccess preamble is received on one among the plurality of random accesschannel resources.
 14. The method according to claim 13, wherein theconfiguration information is included in system information.
 15. Themethod according to claim 13, wherein the plurality of synchronizationsignals are distinguished by different indices, other than a frameindex, a subframe index and a symbol index.
 16. The method according toclaim 15, further comprising: transmitting a random access responseincluding index information regarding one among the different indicesbased on receiving the random access preamble; and performing downlinkchannel transmission or uplink channel reception based on the indexinformation.